Load control device for a light-emitting diode light source

ABSTRACT

A load control device for controlling the amount of power delivered to an electrical load is able to operate in a burst mode to adjust the amount of power delivered to the electrical load to low levels. The load control device comprises a control circuit that operates in a normal mode to regulate an average magnitude of a load current conducted through the load to a target load current that ranges from a maximum rated current to a minimum rated current. The control circuit operates in the burst mode to regulate the average magnitude of the load current below the minimum rated current. During the burst mode, the control circuit regulates a peak magnitude of the load current to the minimum rated current during a first time period, and stops regulating the load current during a second time period, such that the average magnitude of the load current is below the minimum rated current.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Non-Provisional applicationSer. No. 14/536,491, filed on Nov. 7, 2014 which claims the benefit ofU.S. Provisional Application No. 62/032,229 filed on Aug. 1, 2014, andU.S. Provisional Application No. 61/901,480 filed on Nov. 8, 2013, whichare incorporated by referenced herein in their entirety.

BACKGROUND

Light-emitting diode (LED) light sources (i.e., LED light engines) areoften used in place of or as replacements for conventional incandescent,fluorescent, or halogen lamps, and the like. LED light sources maycomprise a plurality of light-emitting diodes mounted on a singlestructure and provided in a suitable housing. LED light sources aretypically more efficient and provide longer operational lives ascompared to incandescent, fluorescent, and halogen lamps. In order toilluminate properly, an LED driver control device (i.e., an LED driver)must be coupled between an alternating-current (AC) source and the LEDlight source for regulating the power supplied to the LED light source.The LED driver may regulate either the voltage provided to the LED lightsource to a particular value, the current supplied to the LED lightsource to a specific peak current value, or may regulate both thecurrent and voltage.

LED light sources are typically rated to be driven via one of twodifferent control techniques: a current load control technique or avoltage load control technique. An LED light source that is rated forthe current load control technique is also characterized by a ratedcurrent (e.g., approximately 350 milliamps) to which the peak magnitudeof the current through the LED light source should be regulated toensure that the LED light source is illuminated to the appropriateintensity and color. In contrast, an LED light source that is rated forthe voltage load control technique is characterized by a rated voltage(e.g., approximately 15 volts) to which the voltage across the LED lightsource should be regulated to ensure proper operation of the LED lightsource. Typically, each string of LEDs in an LED light source rated forthe voltage load control technique includes a current balance regulationelement to ensure that each of the parallel legs has the same impedanceso that the same current is drawn in each parallel string.

It is known that the light output of an LED light source can be dimmed.Different methods of dimming LEDs include a pulse-width modulation (PWM)technique and a constant current reduction (CCR) technique. Pulse-widthmodulation dimming can be used for LED light sources that are controlledin either a current or voltage load control mode/technique. Inpulse-width modulation dimming, a pulsed signal with a varying dutycycle is supplied to the LED light source. If an LED light source isbeing controlled using the current load control technique, the peakcurrent supplied to the LED light source is kept constant during an ontime of the duty cycle of the pulsed signal. However, as the duty cycleof the pulsed signal varies, the average current supplied to the LEDlight source also varies, thereby varying the intensity of the lightoutput of the LED light source. If the LED light source is beingcontrolled using the voltage load control technique, the voltagesupplied to the LED light source is kept constant during the on time ofthe duty cycle of the pulsed signal in order to achieve the desiredtarget voltage level, and the duty cycle of the load voltage is variedin order to adjust the intensity of the light output. Constant currentreduction dimming is typically only used when an LED light source isbeing controlled using the current load control technique. In constantcurrent reduction dimming, current is continuously provided to the LEDlight source, however, the DC magnitude of the current provided to theLED light source is varied to thus adjust the intensity of the lightoutput. Examples of LED drivers are described in greater detail incommonly-assigned U.S. Pat. No. 8,492,987, issued Jul. 23, 2010, andU.S. Patent Application Publication No. 2013/0063047, published Mar. 14,2013, both entitled LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODE LIGHTSOURCE, the entire disclosures of which are hereby incorporated byreference.

SUMMARY

As described herein, a load control device for controlling the amount ofpower delivered to an electrical load is able to operate in a burst modeto adjust the amount of power delivered to the electrical load to lowlevels. The load control device may comprise a load regulation circuitconfigured to control a magnitude of a load current conducted throughthe electrical load to control the amount of power delivered to theelectrical load. The load control device may comprise a controllercoupled to the load regulation circuit and configured to adjust anaverage magnitude of the load current. The controller may be configuredto operate in a normal mode and a burst mode. In the normal mode, thecontroller may be configured to regulate the average magnitude of theload current to a target load current. The target load current may rangefrom a maximum rated current to a minimum rated current. In the burstmode, the controller may be configured to regulate the average magnitudeof the load current below the minimum rated current. For example, duringthe burst mode, the controller may be configured to regulate a peakmagnitude of the load current to the minimum rated current during afirst time period using a control loop, and to stop regulating the loadcurrent during a second time period, such that the average magnitude ofthe load current is below the minimum rated current. The control circuitmay be configured to freeze the control loop during the second timeperiod.

An LED driver for controlling the intensity of an LED light source maybe provided. The LED driver may comprise an LED drive circuit and acontroller. The LED drive circuit may be operable to conduct a loadcurrent through the LED light source and to control the intensity of theLED light source. The controller may be coupled to the LED drive circuitfor adjusting an average magnitude of the load current to control theintensity of the lighting load to a target intensity. The controller maybe configured to operate in a normal mode and a burst mode. In thenormal mode, the controller may be configured to regulate the averagemagnitude of the load current to a target load current. The target loadcurrent may range from a maximum rated current to a minimum ratedcurrent. In the burst mode, the controller may be configured to regulatethe average magnitude of the load current below the minimum ratedcurrent. During the burst mode, the controller may be configured tocontrol the load regulation circuit in an active state and an inactivestate. For example, the controller may be configured to control the loadregulation circuit in an active state during a first time period inwhich the controller regulates a peak magnitude of the load current tothe minimum rated current using a control loop. The controller may beconfigured to control the load regulation circuit in the inactive stateduring a second time period in which the controller freezes the controlloop to stop regulating the load current, such that the averagemagnitude of the load current is below the minimum rated current.

A load control device for controlling the amount of power delivered toan electrical load may be provided herein. The load control device maycomprise a load regulation circuit and a control circuit. The loadregulation circuit may be configured to control a magnitude of a loadcurrent conducted through the electrical load to control the amount ofpower delivered to the electrical load. The control circuit may becoupled to the load regulation circuit and configured to adjust anaverage magnitude of the load current. The control circuit may beconfigured to operate in a normal mode and a burst mode. The burst modemay comprise an active state during a first time period of a burst modeperiod and an inactive state during a second time period of the burstmode period. The duration of the active state of the burst mode periodis determined based on a burst duty cycle. During the normal mode, thecontrol circuit may be configured to regulate the average magnitude ofthe load current by holding the burst duty cycle constant and adjustinga target load current. During the burst mode, the control circuit may beconfigured to regulate the average magnitude of the load current byadjusting the burst duty cycle and/or the target load current. Duringburst mode, the control circuit may be configured to adjust the targetload current by a current offset that ranges from a minimum currentoffset to a maximum current offset based on the burst duty cycle and atarget amount of power to be delivered to the electrical load.

In addition, a load control device for controlling the amount of powerdelivered to an electrical load may comprise a load regulation circuitconfigured to control a magnitude of a load current conducted throughthe electrical load to control the amount of power delivered to theelectrical load, and a control circuit coupled to the load regulationcircuit and configured to adjust an average magnitude of the loadcurrent. The control circuit may be configured to operate in a normalmode to regulate the average magnitude of the load current between amaximum rated current to a minimum rated current. The control circuitmay be further configured to operate in a burst mode to regulate theaverage magnitude of the load current below the minimum rated current.During the burst mode, the control circuit may be configured to regulatea peak magnitude of the load current to a target load current during afirst time period, and to stop regulating the load current during asecond time period resulting in the average magnitude of the loadcurrent being below the minimum rated current. The control circuit maybe configured to increase the magnitude of the load current from aninitial current to the target load current over a ramp time period atthe beginning of the first time period.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a light-emitting diode (LED)driver for controlling the intensity of an LED light source.

FIG. 2 is an example plot of a target load current of the LED driver ofFIG. 1 as a function of a target intensity.

FIG. 3 is an example plot of a burst duty cycle of the LED driver ofFIG. 1 as a function of the target intensity.

FIG. 4 is an example state diagram illustrating the operation of a loadregulation circuit of the LED driver of FIG. 1 when operating in a burstmode.

FIG. 5 is a simplified schematic diagram of an isolated forwardconverter and a current sense circuit of an LED driver.

FIG. 6 is an example diagram illustrating a magnetic core set of anenergy-storage inductor of a forward converter.

FIG. 7 shows example waveforms illustrating the operation of a forwardconverter and a current sense circuit when the intensity of an LED lightsource is near a high-end intensity.

FIG. 8 shows example waveforms illustrating the operation of a forwardconverter and a current sense circuit when the intensity of an LED lightsource is near a low-end intensity.

FIG. 9 is an example plot of a relationship between an offset time and atarget intensity of an LED driver.

FIG. 10 is an example plot of an alternative relationship between theoffset time and the target intensity of an LED driver.

FIG. 11 shows example waveforms illustrating the operation of a forwardconverter of an LED driver when operating in a burst mode.

FIG. 12A is a diagram of an example waveform illustrating the loadcurrent I_(LOAD) when a load regulation circuit is operating in a burstmode.

FIG. 12B is a diagram of an example waveform illustrating the loadcurrent I_(LOAD) when a load regulation circuit is operating in a burstmode.

FIG. 12C is example waveforms illustrating an example of how a loadcontrol circuit may determine the current offset when holding the activestate period constant during burst mode.

FIG. 13 is an example of a plot relationship between a target loadcurrent and the burst duty cycle, and the target intensity of a lightsource.

FIG. 14A is an example waveform illustrating an overshoot in a loadcurrent conducted through a light source.

FIG. 14B is an example waveform illustrating control of a rise time of aload current conducted through a light source at the beginning of eachactive state period during burst mode.

FIGS. 15A, 15B, 16, 17A, 17B, and 18 are simplified flowcharts ofexample procedures for operating a forward converter of an LED driver ina normal mode and a burst mode.

DETAILED DESCRIPTION

FIG. 1 is a simplified block diagram of a load control device, e.g., alight-emitting diode (LED) driver 100, for controlling the amount ofpower delivered to an electrical load, such as, an LED light source 102(e.g., an LED light engine), and thus the intensity of the electricalload. The LED light source 102 is shown as a plurality of LEDs connectedin series but may comprise a single LED or a plurality of LEDs connectedin parallel or a suitable combination thereof, depending on theparticular lighting system. The LED light source 102 may comprise one ormore organic light-emitting diodes (OLEDs). The LED driver 100 maycomprise a hot terminal H and a neutral terminal that are adapted to becoupled to an alternating-current (AC) power source (not shown).

The LED driver 100 may comprise a radio-frequency (RFI) filter circuit110, a rectifier circuit 120, a boost converter 130, a load regulationcircuit 140, a control circuit 150, a current sense circuit 160, amemory 170, a communication circuit 180, and/or a power supply 190. TheRFI filter circuit 110 may minimize the noise provided on the AC mains.The rectifier circuit 120 may generate a rectified voltage V_(RECT).

The boost converter 130 may receive the rectified voltage V_(RECT) andgenerate a boosted direct-current (DC) bus voltage V_(BUS) across a buscapacitor C_(BUS). The boost converter 130 may comprise any suitablepower converter circuit for generating an appropriate bus voltage, suchas, for example, a flyback converter, a single-ended primary-inductorconverter (SEPIC), a Ćuk converter, or other suitable power convertercircuit. The boost converter 120 may operate as a power factorcorrection (PFC) circuit to adjust the power factor of the LED driver100 towards a power factor of one.

The load regulation circuit 140 may receive the bus voltage V_(BUS) andcontrol the amount of power delivered to the LED light source 102, forexample, to control the intensity of the LED light source 102 between alow-end (i.e., minimum) intensity L_(LE) (e.g., approximately 1-5%) anda high-end (i.e., maximum) intensity L_(HE) (e.g., approximately 100%).An example of the load regulation circuit 140 may be an isolated,half-bridge forward converter. An example of the load control device(e.g., LED driver 100) comprising a forward converter is described ingreater detail in commonly-assigned U.S. patent application Ser. No.13/935,799, filed Jul. 5, 2013, entitled LOAD CONTROL DEVICE FOR ALIGHT-EMITTING DIODE LIGHT SOURCE, the entire disclosure of which ishereby incorporated by reference. The load regulation circuit 140 maycomprise, for example, a buck converter, a linear regulator, or anysuitable LED drive circuit for adjusting the intensity of the LED lightsource 102.

The control circuit 150 may be configured to control the operation ofthe boost converter 130 and/or the load regulation circuit 140. Anexample of the control circuit 150 may be a controller. The controlcircuit 150 may comprise, for example, a digital controller or any othersuitable processing device, such as, for example, a microcontroller, aprogrammable logic device (PLD), a microprocessor, an applicationspecific integrated circuit (ASIC), or a field-programmable gate array(FPGA). The control circuit 150 may generate a bus voltage controlsignal V_(BUS-CNTL), which may be provided to the boost converter 130for adjusting the magnitude of the bus voltage V_(BUS). The controlcircuit 150 may receive a bus voltage feedback control signal V_(BUS-FB)from the boost converter 130, which may indicate the magnitude of thebus voltage V_(BUS).

The control circuit 150 may generate drive control signals V_(DRIVE1),V_(DRIVE2). The drive control signals V_(DRIVE1), V_(DRIVE2) may beprovided to the load regulation circuit 140 for adjusting the magnitudeof a load voltage V_(LOAD) generated across the LED light source 102 andthe magnitude of a load current I_(LOAD) conducted through the LED lightsource 120, for example, to control the intensity of the LED lightsource 120 to a target intensity L_(TRGT). The control circuit 150 mayadjust an operating frequency f_(OP) and/or a duty cycle DC_(INV) (e.g.,an on time T_(ON)) of the drive control signals V_(DRIVE1), V_(DRIVE2)to adjust the magnitude of the load voltage V_(LOAD) and/or the loadcurrent I_(LOAD).

The current sense circuit 160 may receive a sense voltage V_(SENSE)generated by the load regulation circuit 140. The sense voltageV_(SENSE) may indicate the magnitude of the load current I_(LOAD). Thecurrent sense circuit 160 may receive a signal-chopper control signalV_(CHOP) from the control circuit 150. The current sense circuit 160 maygenerate a load current feedback signal V_(I-LOAD), which may be a DCvoltage indicating the average magnitude I_(AVE) of the load currentI_(LOAD). The control circuit 150 may receive the load current feedbacksignal V_(I-LOAD) from the current sense circuit 160 and control thedrive control signals V_(DRIVE1), V_(DRIVE2) accordingly. For example,the control circuit 150 may control the drive control signalsV_(DRIVE1), V_(DRIVE2) to adjust a magnitude of the load currentI_(LOAD) to a target load current I_(TRGT) to thus control the intensityof the LED light source 102 to the target intensity L_(TRGT) (e.g.,using a control loop).

The load current I_(LOAD) may be the current that is conducted throughthe LED light source 120. The target load current I_(TRGT) may be thecurrent that the control circuit 150 would ideally like to conductthrough the LED light source 120 (e.g., based at least on the loadcurrent feedback signal V_(I-LOAD)). The control circuit 150 may belimited to specific levels of granularity in which it can control thecurrent conducted through the LED light source 120 (e.g., due toinverter cycle lengths, etc.), so the control circuit 150 may not alwaysbe able to achieve the target load current I_(TRGT). For example, FIGS.2 and 13 illustrate the current conducted through an LED light source asa linear graph (at least in parts), and as such, illustrate the targetload current I_(TRGT) since the load current I_(LOAD) itself may notactually follow a true linear path. Further, non-ideal reactions of theLED light source 120 (e.g., an overshoot in the load current I_(LOAD),for example, as shown in FIG. 14A) may cause the load current I_(LOAD)to deviate from the target load current I_(TRGT). In the idealsituation, the load current I_(LOAD) is approximately equal to thetarget load current I_(TRGT).

The control circuit 150 may be coupled to the memory 170. The memory 170may store operational characteristics of the LED driver 100 (e.g., thetarget intensity L_(TRGT), the low-end intensity L_(LE), the high-endintensity L_(HE), etc.). The communication circuit 180 may be coupledto, for example, a wired communication link or a wireless communicationlink, such as a radio-frequency (RF) communication link or an infrared(IR) communication link. The control circuit 150 may be configured toupdate the target intensity L_(TRGT) of the LED light source 102 and/orthe operational characteristics stored in the memory 170 in response todigital messages received via the communication circuit 180. The LEDdriver 100 may be operable to receive a phase-control signal from adimmer switch for determining the target intensity L_(TRGT) for the LEDlight source 102. The power supply 190 may receive the rectified voltageV_(RECT) and generate a direct-current (DC) supply voltage V_(CC) forpowering the circuitry of the LED driver 100.

FIG. 2 is an example plot of the target load current I_(TRGT) as afunction of the target intensity L_(TRGT). The magnitude of the loadcurrent I_(LOAD) may only be regulated to values between a maximum ratedcurrent I_(MAX) and a minimum rated current I_(MIN), for example, due tohardware limitations of the load regulation circuit 140 and the controlcircuit 150. Thus, the target load current I_(TRGT) may only be adjustedbetween the maximum rated current I_(MAX) and the minimum rated currentI_(MIN). When the target intensity L_(TRGT) is between the high-endintensity L_(HE) (e.g., approximately 100%) and a transition intensityL_(TRAN) (e.g., approximately 5%), the control circuit 150 may operatethe load regulation circuit 140 in a normal mode in which an averagemagnitude I_(AVE) of the load current I_(LOAD) is controlled to be equalto the target load current I_(TRGT). In the normal mode, the controlcircuit 150 may adjust the average magnitude I_(AVE) of the load currentI_(LOAD) to the target load current I_(TRGT) in response to the loadcurrent feedback signal V_(I-LOAD), e.g., using closed loop control. Thecontrol circuit 150 may adjust the target load current I_(TRGT) betweenthe maximum rated current I_(MAX) and the minimum rated current I_(MIN)in the normal mode, for example, as shown in FIG. 2.

FIG. 3 is an example plot of a burst duty cycle DC_(BURST) (e.g., anideal burst duty cycle DC_(BURST-IDEAL)) as a function of the targetintensity L_(TRGT). When the target intensity L_(TRGT) is between thehigh-end intensity L_(HE) (e.g., approximately 100%) and a transitionintensity L_(TRAN) (e.g., approximately 5%), the control circuit 150 maybe configured to operate the load regulation circuit 140 to set theburst duty cycle DC_(BURST) equal to a maximum duty cycle DC_(MAX)(e.g., approximately 100%). To adjust the target intensity L_(TRGT)below the transition intensity L_(TRAN), the control circuit 150 may beconfigured to operate the load regulation circuit 140 in a burst mode toreduce the average magnitude I_(AVE) of the load current I_(LOAD) to beless the minimum rated current I_(MIN). For example, to adjust thetarget intensity L_(TRGT) below the transition intensity L_(TRAN), thecontrol circuit 150 may be configured to operate the load regulationcircuit 140 to reduce the burst duty cycle DC_(BURST) below the maximumduty cycle DC_(MAX). For example, the load regulation circuit 140 mayadjust the burst duty cycle DC_(BURST) between the maximum duty cycleDC_(MAX) (e.g., approximately 100%) and a minimum duty cycle DC_(MIN)(e.g., approximately 20%). In the burst mode, a peak magnitude I_(PK) ofthe load current I_(LOAD) may be equal to the target current I_(TRGT)(e.g., the minimum rated current I_(MIN)). For example, the peakmagnitude I_(PK) of the load current I_(LOAD) may be equal to theminimum rated current I_(MIN) during an active state of the burst mode.

With reference to FIG. 3, the burst duty cycle DC_(BURST) may refer toan ideal burst duty cycle DC_(BURST-IDEAL), which may include an integerportion DC_(BURST-INTEGER) and/or a fractional portionDC_(BURST-FRACTIONAL). The integer portion DC_(BURST-INTEGER) may becharacterized by the percentage of the ideal burst duty cycleDC_(BURST-IDEAL) that includes complete inverter cycles (i.e., aninteger value of inverter cycles). The fractional portionDC_(BURST-FRACTIONAL) may be characterized by the percentage of theideal burst duty cycle DC_(BURST-IDEAL) that includes a fraction of aninverter cycle. As described herein, the control circuit 150 (e.g., viathe load regulation circuit 140) may be configured to adjust the numberof inverter cycles only by an integer number (i.e., byDC_(BURST-INTEGER)) and not a fractional amount (i.e.,DC_(BURST-FRACTIONAL)). Therefore, the example plot of FIG. 3 mayillustrate an ideal curve showing the adjustment of the ideal burst dutycycle DC_(BURST-IDEAL) from a maximum duty cycle DC_(MAX) to a minimumduty cycle DC_(MIN) when the target intensity L_(TRGT) is below thetransition intensity L_(TRAN). Nonetheless, unless defined differently,burst duty cycle DC_(BURST) may refer to the integer portionDC_(BURST-INTEGER) of the ideal burst duty cycle DC_(BURST-IDEAL), forexample, if the control circuit 150 is not be configured to operate theburst duty cycle DC_(BURST) at fractional amounts.

FIG. 4 is an example state diagram illustrating the operation of theload regulation circuit 140 in the burst mode. During the burst mode,the control circuit 150 may periodically control the load regulationcircuit 140 into an active state and an inactive state, e.g., independence upon a burst duty cycle DC_(BURST) and a burst mode periodT_(BURST) (e.g., approximately 4.4 milliseconds). For example, theactive state period (T_(ACTIVE)) may be equal to the burst duty cycle(DC_(BURST)) times the burst mode period (T_(BURST)) and the inactivestate period (T_(INACTIVE)) may be equal to one minus the burst dutycycle (DC_(BURST)) times the burst mode period (T_(BURST)). That is,T_(ACTIVE)=DC_(BURST)·T_(BURST) and T_(INACTIVE)(1−DC_(BURST))·T_(BURST).

In the active state of the burst mode, the control circuit 150 maygenerate (e.g., actively generate) the drive control signals V_(DRIVE1),V_(DRIVE2) to adjust the magnitude (e.g., the peak magnitude I_(PK)) ofthe load current I_(LOAD), e.g., using closed loop control. For example,in the active state of the burst mode, the control circuit 150 maygenerate the drive signals V_(DRIVE1), V_(DRIVE2) to adjust themagnitude of the load current I_(LOAD) to be equal to a target loadcurrent I_(TRGT) (e.g., the minimum rated current I_(MIN)) in responseto the load current feedback signal V_(I-LOAD).

In the inactive state of the burst mode, the control circuit 150 mayfreeze the control loop and may not generate the drive control signalsV_(DRIVE1), V_(DRIVE2), for example, such that the magnitude of the loadcurrent I_(LOAD) drops to approximately zero amps. While the controlloop is frozen (e.g., in the inactive state), the control circuit 150may not adjust the values of the operating frequency f_(OP) and/or theduty cycle DC_(INV) in response to the load current feedback signalV_(I-LOAD) (e.g., even though the control circuit 150 is not presentlygenerating the drive signals V_(DRIVE1), V_(DRIVE2)). For example, thecontrol circuit 150 may store the present duty cycle DC_(INV) (e.g., thepresent on time T_(ON)) of the drive control signals V_(DRIVE1),V_(DRIVE2) in the memory 170 prior to (e.g., immediately prior to)freezing the control loop. Accordingly, when the control loop isunfrozen (e.g., when the control circuit 150 enters the active state),the control circuit 150 may continue to generate the drive controlsignals V_(DRIVE1), V_(DRIVE2) using the operating frequency f_(OP)and/or the duty cycle DC_(INV) from the previous active state.

The control circuit 150 may be configured to adjust the burst duty cycleDC_(BURST) using an open loop control. For example, the control circuit150 may be configured to adjust the burst duty cycle DC_(BURST) as afunction of the target intensity L_(TRGT), for example, when the targetintensity L_(TRGT) is below the transition intensity L_(TRAN). Thecontrol circuit 150 may be configured to linearly decrease the burstduty cycle DC_(BURST) as the target intensity L_(TRGT) is decreasedbelow the transition intensity L_(TRAN) (e.g., as shown in FIG. 3),while the target load current I_(TRGT) is held constant at the minimumrated current I_(MIN) (e.g., as shown in FIG. 2). Since the controlcircuit 150 changes between the active state and the inactive state independence upon the burst duty cycle DC_(BURST) and the burst modeperiod T_(BURST) (e.g., as shown in the state diagram of FIG. 4), theaverage magnitude I_(AVE) of the load current I_(LOAD) may be a functionof the burst duty cycle DC_(BURST) (e.g., I_(AVE)=DC_(BURST)·I_(MIN)).During the burst mode, the peak magnitude I_(PK) of the load currentI_(LOAD) may be equal to the minimum rated current I_(MIN), but theaverage magnitude I_(AVE) of the load current I_(LOAD) may be less thanthe minimum rated current I_(MIN).

FIG. 5 is a simplified schematic diagram of a forward converter 240 anda current sense circuit 260 of an LED driver (e.g., the LED driver 100shown in FIG. 1). The forward converter 240 may be an example of theload regulation circuit 140 of the LED driver 100 shown in FIG. 1. Thecurrent sense circuit 260 may be an example of the current sense circuit160 of the LED driver 100 shown in FIG. 1.

The forward converter 240 may comprise a half-bridge inverter circuithaving two field effect transistors (FETs) Q210, Q212 for generating ahigh-frequency inverter voltage V_(INV) from the bus voltage V_(BUS).The FETs Q210, Q212 may be rendered conductive and non-conductive inresponse to the drive control signals V_(DRIVE1), V_(DRIVE2). The drivecontrol signals V_(DRIVE1), V_(DRIVE2) may be received from the controlcircuit 150. The drive control signals V_(DRIVE1), V_(DRIVE2) may becoupled to the gates of the respective FETs Q210, Q212 via a gate drivecircuit 214 (e.g., which may comprise part number L6382DTR, manufacturedby ST Microelectronics). The control circuit 150 may generate theinverter voltage V_(INV) at a constant operating frequency f_(OP) (e.g.,approximately 60-65 kHz) and thus a constant operating period T_(OP).However, the operating frequency f_(OP) may be adjusted under certainoperating conditions. The control circuit 150 may be configured toadjust a duty cycle DC_(INV) of the inverter voltage V_(INV) to controlthe intensity of an LED light source 202 towards the target intensityL_(TRGT).

In a normal mode of operation, when the target intensity L_(TRGT) of theLED light source 202 is between the high-end intensity L_(HE) and thetransition intensity L_(TRAN), the control circuit 150 may adjust theduty cycle DC_(INV) of the inverter voltage V_(INV) to adjust themagnitude (e.g., the average magnitude I_(AVE)) of the load currentI_(LOAD) towards the target load current I_(TRGT). As previouslymentioned, the magnitude of the load current I_(LOAD) may vary betweenthe maximum rated current I_(MAX) and the minimum rated current I_(MIN)(e.g., as shown in FIG. 2). At the minimum rated current I_(MIN) (e.g.,at the transition intensity L_(TRAN)), the inverter voltage V_(INV) maybe characterized by a transition operating frequency f_(OP-T), atransition operating period T_(OP-T), and a transition duty cycleDC_(INV-T).

When the target intensity L_(TRGT) of the LED light source 202 is belowthe transition intensity L_(TRAN), the control circuit 150 may beconfigured to operate the forward converter 240 in a burst mode ofoperation. In one or more embodiments, the control circuit 150 may usepower (e.g., a transition power) and/or current (e.g., a transitioncurrent) as a threshold to determine when to operate in burst mode(e.g., instead of intensity). In the burst mode of operation, thecontrol circuit 150 may be configured to switch the forward converter240 between an active mode (e.g., in which the control circuit 150actively generates the drive control signals V_(DRIVE1), V_(DRIVE2) toregulate the peak magnitude I_(PK) of the load current I_(LOAD) to beequal to the minimum rated current I_(MIN)) and an inactive mode (e.g.,in which the control circuit 150 freezes the control loop and does notgenerate the drive control signals V_(DRIVE1), V_(DRIVE2)), for example,as shown in the state diagram of FIG. 4. In the burst mode, the controlcircuit 150 may change the forward converter 240 between the activestate and the inactive state in dependence upon a burst duty cycleDC_(BURST) and a burst mode period T_(BURST) (e.g., as shown in FIG. 4)and adjust the burst duty cycle DC_(BURST) as a function of the targetintensity L_(TRGT), which is below the transition intensity L_(TRAN)(e.g., as shown in FIG. 3). In the normal mode and in the active stateof the burst mode, the forward converter 240 may be characterized by aturn-on time T_(TURN-ON) and a turn-off time T_(TURN-OFF). The turn-ontime T_(TURN-ON) may be a time period from when the drive controlsignals V_(DRIVE1), V_(DRIVE2) are driven until the respective FET Q210,Q212 is rendered conductive. The turn-off time T_(TURN-OFF) may be atime period from when the drive control signals V_(DRIVE1), V_(DRIVE2)are driven until the respective FET Q210, Q212 is renderednon-conductive.

The inverter voltage V_(INV) is coupled to the primary winding of atransformer 220 through a DC-blocking capacitor C216 (e.g., which mayhave a capacitance of approximately 0.047 g), such that a primaryvoltage V_(PRI) is generated across the primary winding. The transformer220 may be characterized by a turns ratio n_(TURNS) (i.e., N₁/N₂), whichmay be approximately 115:29. A sense voltage V_(SENSE) may be generatedacross a sense resistor 8222, which may be coupled in series with theprimary winding of the transformer 220. The FETs Q210, Q212 and theprimary winding of the transformer 220 may be characterized by parasiticcapacitances C_(P1), C_(P2), C_(P3), respectively. The secondary windingof the transformer 220 may generate a secondary voltage. The secondaryvoltage may be coupled to the AC terminals of a full-wave dioderectifier bridge 224 for rectifying the secondary voltage generatedacross the secondary winding. The positive DC terminal of the rectifierbridge 224 may be coupled to the LED light source 202 through an outputenergy-storage inductor L226 (e.g., which may have an inductance ofapproximately 10 mH), such that the load voltage V_(LOAD) may begenerated across an output capacitor C228 (e.g., which may have acapacitance of approximately 3 g).

The current sense circuit 260 may comprise an averaging circuit forproducing the load current feedback signal V_(I-LOAD). The averagingcircuit may comprise a low-pass filter comprising a capacitor C230(e.g., which may have a capacitance of approximately 0.066 uF) and aresistor R232 (e.g., which may have a resistance of approximately 3.32kΩ). The low-pass filter may receive the sense voltage V_(SENSE) via aresistor R234 (e.g., which may have a resistance of approximately 1 kΩ).The current sense circuit 160 may comprise a transistor Q236 (e.g., aFET as shown in FIG. 5) coupled between the junction of the resistorsR232, R234 and circuit common. The gate of the transistor Q236 may becoupled to circuit common through a resistor R238 (e.g., which may havea resistance of approximately 22 kΩ). The gate of the transistor Q236may receive the signal-chopper control signal V_(CHOP) from the controlcircuit 150. An example of the current sense circuit 260 may bedescribed in greater detail in commonly-assigned U.S. patent applicationSer. No. 13/834,153, filed Mar. 15, 2013, entitled FORWARD CONVERTERHAVING A PRIMARY-SIDE CURRENT SENSE CIRCUIT, the entire disclosure ofwhich is hereby incorporated by reference.

FIG. 6 is an example diagram illustrating a magnetic core set 290 of anenergy-storage inductor (e.g., the output energy-storage inductor L226of the forward converter 240 shown in FIG. 5). The magnetic core set 290may comprise two E-cores 292A, 292B, and may comprise part numberPC40EE16-Z, manufactured by TDK Corporation. The E-cores 292A, 292B maycomprise respective outer legs 294A, 294B and inner legs 296A, 296B.Each inner leg 296A, 296B may be characterized by a width w_(LEG) (e.g.,approximately 4 mm). The inner leg 296A of the first E-core 292A maycomprise a partial gap 298A (i.e., the magnetic core set 290 ispartially-gapped), such that the inner legs 296A, 296B are spaced apartby a gap distance d_(GAP) (e.g., approximately 0.5 mm). The partial gap298A may extend for a gap width w_(GAP) (e.g., approximately 2.8 mm)such that the partial gap 298A extends for approximately 70% of the legwidth w_(LEG) of the inner leg 296A. In one or more embodiments, both ofthe inner legs 296A, 296B may comprise partial gaps. Thepartially-gapped magnetic core set 290 (e.g., as shown in FIG. 6) mayallow the output energy-storage inductor L226 of the forward converter240 (e.g., shown in FIG. 5) to maintain continuous current at low loadconditions (e.g., near the low-end intensity L_(LE)).

FIG. 7 shows example waveforms illustrating the operation of a forwardconverter and a current sense circuit, for example, the forwardconverter 240 and the current sense circuit 260 shown in FIG. 5. Forexample, the forward converter 240 may generate the waveforms shown inFIG. 7 when operating in the normal mode and in the active state of theburst mode as described herein. As shown in FIG. 7, a control circuit(e.g., the control circuit 150) may drive the respective drive controlsignals V_(DRIVE1), V_(DRIVE2) high to approximately the supply voltageV_(CC) to render the respective FETs Q210, Q212 conductive for an ontime T_(ON) at different times (i.e., the FETs Q210, Q212 are conductiveat different times). When the high-side FET Q210 is conductive, theprimary winding of the transformer 220 may conduct a primary currentI_(PRI) to circuit common through the capacitor C216 and sense resistorR222. After (e.g., immediately after) the high-side FET Q210 is renderedconductive (at time t₁ in FIG. 7), the primary current I_(PRI) mayconduct a short high-magnitude pulse of current due to the parasiticcapacitance C_(P3) of the transformer 220 as shown in FIG. 7. While thehigh-side FET Q210 is conductive, the capacitor C216 may charge, suchthat a voltage having a magnitude of approximately half of the magnitudeof the bus voltage V_(BUS) is developed across the capacitor.Accordingly, the magnitude of the primary voltage V_(PRI) across theprimary winding of the transformer 220 may be equal to approximatelyhalf of the magnitude of the bus voltage V_(BUS) (i.e., V_(BUS)/2). Whenthe low-side FET Q212 is conductive, the primary winding of thetransformer 220 may conduct the primary current I_(PRI) in an oppositedirection and the capacitor C216 may be coupled across the primarywinding, such that the primary voltage V_(PRI) may have a negativepolarity with a magnitude equal to approximately half of the magnitudeof the bus voltage V_(BUS).

When either of the high-side and low-side FETs Q210, Q212 areconductive, the magnitude of an output inductor current I_(L) conductedby the output inductor L226 and the magnitude of the load voltageV_(LOAD) across the LED light source 202 may increase with respect totime. The magnitude of the primary current I_(PRI) may increase withrespect to time while the FETs Q210, Q212 are conductive (e.g., after aninitial current spike). When the FETs Q210, Q212 are non-conductive, theoutput inductor current I_(L) and the load voltage V_(LOAD) may decreasein magnitude with respective to time. The output inductor current I_(L)may be characterized by a peak magnitude I_(L-PK) and an averagemagnitude I_(L-AVG), for example, as shown in FIG. 7. The controlcircuit 150 may increase and/or decrease the on times T_(ON) of thedrive control signals V_(DRIVE1), V_(DRIVE2) (e.g., and the duty cycleDC_(INV) of the inverter voltage V_(INV)) to respectively increase anddecrease the average magnitude I_(L-AVG) of the output inductor currentI_(L), and thus respectively increase and decrease the intensity of theLED light source 202.

When the FETs Q210, Q212 are rendered non-conductive, the magnitude ofthe primary current I_(PRI) may drop toward zero amps (e.g., as shown attime t₂ in FIG. 7 when the high-side FET Q210 is renderednon-conductive). However, a magnetizing current I_(MAG) may continue toflow through the primary winding of the transformer 220 due to themagnetizing inductance L_(MAG) of the transformer. When the targetintensity L_(TRGT) of the LED light source 102 is near the low-endintensity L_(LE), the magnitude of the primary current I_(PRI) mayoscillate after either of the FETs Q210, Q212 is renderednon-conductive, for example, due to the parasitic capacitances C_(P1),C_(P2) of the FETs, the parasitic capacitance C_(P3) of the primarywinding of the transformer 220, and/or any other parasitic capacitancesof the circuit, such as, parasitic capacitances of the printed circuitboard on which the forward converter 240 is mounted.

The real component of the primary current I_(PRI) may indicate themagnitude of the secondary current I_(SEC) and thus the intensity of theLED light source 202. However, the magnetizing current I_(MAG) (i.e.,the reactive component of the primary current I_(PRI)) may also flowthrough the sense resistor 8222. The magnetizing current I_(MAG) maychange from a negative polarity to a positive polarity when thehigh-side FET Q210 is conductive, change from a positive polarity to anegative polarity when the low-side FET Q212 is conductive, and remainconstant when the magnitude of the primary voltage V_(PRI) is zerovolts, for example, as shown in FIG. 7. The magnetizing current I_(MAG)may have a maximum magnitude defined by the following equation:

$I_{{MAG} - {MAX}} = {\frac{V_{BUS} \cdot T_{HC}}{4 \cdot L_{MAG}}.}$

where T_(HC) may be the half-cycle period of the inverter voltageV_(INV), i.e., T_(HC)=T_(OP)/2. As shown in FIG. 7, the areas 250, 252are approximately equal, such that the average value of the magnitude ofthe magnetizing current I_(MAG) is zero during the period of time whenthe magnitude of the primary voltage V_(PRI) is greater thanapproximately zero volts (e.g., during the ton time T_(ON) as shown inFIG. 7).

The current sense circuit 260 may determine an average the primarycurrent I_(PRI) during the positive cycles of the inverter voltageV_(INV), i.e., when the high-side FET Q210 is conductive (e.g., duringthe on time T_(ON)). The load current feedback signal V_(I-LOAD), whichmay be generated by the current sense circuit 260, may have a DCmagnitude that is the average value of the primary current I_(PRI) whenthe high-side FET Q210 is conductive. Because the average value of themagnitude of the magnetizing current I_(MAG) is approximately zeroduring the period of time that the high-side FET Q210 is conductive(e.g., during the on time T_(ON)), the load current feedback signalV_(I-LOAD) generated by the current sense circuit indicates the realcomponent (e.g., only the real component) of the primary current I_(PRI)during the on time TON.

When the high-side FET Q210 is rendered conductive, the control circuit150 may drive the signal-chopper control signal V_(CHOP) low towardscircuit common to render the transistor Q236 of the current sensecircuit 260 non-conductive for a signal-chopper time T_(CHOP). Thesignal-chopper time T_(CHOP) may be approximately equal to the on timeT_(ON) of the high-side FET Q210, for example, as shown in FIG. 7. Thecapacitor C230 may charge from the sense voltage V_(SENSE) through theresistors R232, R234 while the signal-chopper control signal V_(CHOP) islow, such that the magnitude of the load current feedback signalV_(I-LOAD) is the average value of the primary current I_(PRI) and thusindicates the real component of the primary current during the time whenthe high-side FET Q210 is conductive. When the high-side FET Q210 is notconductive, the control circuit 150 drives the signal-chopper controlsignal V_(CHOP) high to render the transistor Q236 conductive.Accordingly, the control circuit 150 is able to accurately determine theaverage magnitude of the load current I_(LOAD) from the magnitude of theload current feedback signal V_(I-LOAD) since the effects of themagnetizing current I_(MAG) and the oscillations of the primary currentI_(PRI) on the magnitude of the load current feedback signal V_(I-LOAD)are reduced or eliminated completely.

As the target intensity L_(TRGT) of the LED light source 202 isdecreased towards the low-end intensity L_(LE) and the on times T_(ON)of the drive control signals V_(DRIVE1), V_(DRIVE2) get smaller, theparasitic of the load regulation circuit 140 (i.e., the parasiticcapacitances C_(P1), C_(P2) of the FETs Q210, Q212, the parasiticcapacitance C_(P3) of the primary winding of the transformer 220, and/orother parasitic capacitances of the circuit) may cause the magnitude ofthe primary voltage V_(PRI) to slowly decrease towards zero volts afterthe FETs Q210, Q212 are rendered non-conductive.

FIG. 8 shows example waveforms illustrating the operation of a forwardconverter and a current sense circuit (e.g., the forward converter 240and the current sense circuit 260) when the target intensity L_(TRGT) isnear the low-end intensity L_(LE), and when the forward converter 240 isoperating in the normal mode and the active state of the burst mode. Thegradual drop off in the magnitude of the primary voltage V_(PRI) mayallow the primary winding of the transformer 220 to continue to conductthe primary current I_(PRI), such that the transformer 220 may continueto deliver power to the secondary winding after the FETs Q210, Q212 arerendered non-conductive, for example, as shown in FIG. 8. Themagnetizing current I_(MAG) may continue to increase in magnitude afterthe on time T_(ON) of the drive control signal V_(DRIVE1) (e.g., and/orthe drive control signal V_(DRIVE2)). Accordingly, the control circuit150 may increase the signal-chopper time T_(CHOP) to be greater than theon time T_(ON). For example, the control circuit 150 may increase thesignal-chopper time T_(CHOP) (e.g., during which the signal-choppercontrol signal V_(CHOP) is low) by an offset time T_(OS) when the targetintensity L_(TRGT) of the LED light source 202 is near the low-endintensity L_(LE).

FIG. 9 is an example plot of a relationship between the offset timeT_(OS) and the target intensity L_(TRGT) of the LED light source 202,for example, when the target intensity L_(TRGT) is near the low-endintensity L_(LE), and when the forward converter 240 is operating in thenormal mode and the active state of the burst mode (e.g., as shown inFIG. 8). The control circuit 150 may adjust the value of the offset timeT_(OS) as a function of the target intensity L_(TRGT) of the LED lightsource 202. For example, the control circuit 150 may adjust the value ofthe offset time T_(OS) linearly with respect to the target intensityL_(TRGT) when the target intensity L_(TRGT) is below a thresholdintensity L_(TH) (e.g., approximately 10%) and above a transitionintensity L_(TRAN) (e.g., approximately 5%). Above the thresholdintensity L_(TH), the offset time T_(OS) may be held constant, forexample, at approximately zero microseconds. Below the transitionintensity L_(TRAN), the offset time T_(OS) may be held constant at amaximum offset time T_(OS-MAX), for example, because the target loadcurrent I_(TRGT) may be held constant at the minimum rated currentI_(MIN) below the transition intensity L_(TRAN).

FIG. 10 is an example plot of a relationship (e.g., an alternaterelationship) between the offset time T_(OS) and the target intensityL_(TRGT) of the LED light source 202, for example, when the targetintensity L_(TRGT) is near the low-end intensity L_(LE), and when theforward converter 240 is operating in the normal mode and the activestate of the burst mode (e.g., as shown in FIG. 8). The control circuit150 may adjust the value of the offset time T_(OS) as a function of thetarget intensity L_(TRGT) of the LED light source 202. For example, thecontrol circuit 150 may adjust the value of the offset time T_(OS)between the high-end intensity L_(HE) and the transition intensityL_(TRAN), for example, as shown in FIG. 10. For example, the controlcircuit 150 may use the following equation:

${T_{OS} = \frac{\frac{V_{BUS}}{4} \cdot C_{PARASITIC}}{{\frac{T_{ON} + T_{{OS} - {PREV}}}{T_{HC}} \cdot I_{{MAG} - {MAX}}} + {\frac{K_{RIPPLE}}{n_{TURNS}} \cdot I_{LOAD}}}},$

where T_(OS-PREV) may be the previous value of the offset time.K_(RIPPLE) may be the dynamic ripple ratio of the output inductorcurrent I_(L) (e.g., which may be a function of the load currentI_(LOAD)). For example, K_(RIPPLE) may be determined according to thefollowing equation:

K _(RIPPLE) =I _(L-PK) /I _(L-AVG),

and C_(PARASITIC) may be the total parasitic capacitance between thejunction of the FETs Q210, Q212 and circuit common. Below the transitionintensity L_(TRAN), the offset time T_(OS) may be held constant at amaximum offset time T_(OS-MAX).

FIG. 11 shows example waveforms illustrating the operation of a forwardconverter when operating in a burst mode (e.g., the forward converter240 shown in FIG. 5). The inverter circuit of the forward converter 240may generate the inverter voltage V_(INV) during the active state (e.g.,for length of an active state period T_(ACTIVE) as shown in FIG. 11),for example, such that the magnitude of the load current I_(LOAD) may beregulated to the minimum rated current I_(MIN). The inverter voltageV_(INV) may not be generated during the inactive state, e.g., for aninactive state period T_(INACTIVE). The active state may begin on aperiodic basis at a burst mode period T_(BURST) (e.g., approximately 4.4milliseconds). The active state period T_(ACTIVE) and inactive stateperiod T_(INACTIVE) may be characterized by durations that are dependentupon a burst duty cycle DC_(BURST). For example,T_(ACTIVE)=DC_(BURST)·T_(BURST) andT_(INACTIVE)=(1−DC_(BURST))·T_(BURST). The average magnitude I_(AVE) ofthe load current I_(LOAD) may be dependent on the burst duty cycleDC_(BURST). For example, the average magnitude I_(AVE) of the loadcurrent I_(LOAD) may equal the burst duty cycle DC_(BURST) times theload current I_(LOAD) (e.g., I_(AVE)=DC_(BURST)·I_(LOAD)), which in oneexample may be the minimum load current I_(MIN) (e.g.,I_(AVE)=DC_(BURST)·I_(MIN)).

The burst duty cycle DC_(BURST) may be controlled to adjust the averagemagnitude I_(AVE) of the load current I_(LOAD). For example, the burstmode period T_(BURST) may be held constant and the length of the activestate period T_(ACTIVE) may be varied to adjust the duty cycleDC_(BURST), which in turn may vary the average magnitude I_(AVE) of theload current I_(LOAD). The active state period T_(ACTIVE) may be heldconstant, and the length of burst mode period T_(BURST) may be varied toadjust the burst duty cycle DC_(BURST), which in turn may vary theaverage magnitude I_(AVE) of the load current I_(LOAD). Accordingly, asthe burst duty cycle DC_(BURST) is increased, the average magnitudeI_(AVE) of the load current I_(LOAD) may increase, and as the burst dutycycle DC_(BURST) is decreased, the average magnitude I_(AVE) of the loadcurrent I_(LOAD) may decrease.

FIG. 12A is a diagram of an example waveform 1200 illustrating the loadcurrent I_(LOAD) when a load regulation circuit (e.g., the loadregulation circuit 140) is operating in a burst mode, for example, asthe target intensity L_(TRGT) of a light source (e.g., the LED lightsource 202) is increased (e.g., from the low-end intensity L_(LE)). Acontrol circuit (e.g., the control circuit 150 of the LED driver 100shown in FIG. 1 and/or the control circuit 150 controlling the forwardconverter 240 and the current sense circuit 260 shown in FIG. 5) mayadjust the length of the active state period T_(ACTIVE) of the burstmode period T_(BURST) by adjusting the burst duty cycle DC_(BURST).Adjusting the length of the active state period T_(ACTIVE) may adjustthe average magnitude I_(AVE) of the load current I_(LOAD), and in turnthe intensity of the light source.

The active state period T_(ACTIVE) of the load current I_(LOAD) may havea length that is dependent upon the length of an inverter cycle of theinverter circuit of the load regulation circuit (i.e., the operatingperiod T_(OP)). For example, referring to FIG. 11, the active stateperiod T_(ACTIVE) may comprise six inverter cycles, and as such, has alength that is equal to the duration of the six inverter cycles. Thecontrol circuit may adjust (i.e., increase or decrease) the active stateperiods T_(ACTIVE) by adjusting the number of inverter cycles in theactive state period T_(ACTIVE). As such, the control circuit may adjustthe active state periods T_(ACTIVE) by predetermined time intervals thatcorrespond to the length of an inverter cycle of the inverter circuit ofthe load regulation circuit, for example, the transition operatingperiod T_(OP-T) (e.g., approximately 12.8 microseconds). Therefore, theactive state period T_(ACTIVE) may be characterized by one or moreinverter cycles, and may be adjusted by adjusting a number of invertercycles per active state period T_(ACTIVE). As such, the averagemagnitude I_(AVE) of the load current I_(LOAD) may be adjusted bypredetermined increments, for example, corresponding to a change in loadcurrent I_(LOAD) due to an increase or decrease of an inverter cycle peractive state period T_(ACTIVE).

One or more burst mode periods T_(BURST) of the load regulation circuitmay be characterized by active state periods T_(ACTIVE) that comprisethe same number of inverter cycles. In the example of FIG. 12A, threeburst mode periods T_(BURST) 1202, 1204, 1206 may be characterized byequivalent active state periods T_(ACTIVE1) (i.e., active state periodsT_(ACTIVE1) that have the same number of inverter cycles). The activestate period T_(ACTIVE2) of the burst mode period T_(BURST) 1208 may belarger than the active state periods T_(ACTIVE1) of the other burst modeperiods T_(BURST) 1202, 1204, 1206. In other words, the active stateperiod T_(ACTIVE2) during the burst mode period T_(BURST) 1208 may beincreased as compared to the active state periods T_(ACTIVE1) during theburst mode periods T_(BURST) 1202, 1204, 1206. As such, the averagemagnitude I_(AVE) of the load current I_(LOAD) may be increased inaccordance with the additional inverter cycle(s) of the active stateperiod T_(ACTIVE2) during the burst mode period T_(BURST) 1208.Therefore, the control circuit may adjust (i.e., increase or decrease)the average magnitude I_(AVE) of the load current I_(LOAD) by adjustingthe active state period T_(ACTIVE) by increments of one or more invertercycles.

FIG. 12B is a diagram of an example waveform 1210 illustrating the loadcurrent I_(LOAD) when a load regulation circuit (e.g., the loadregulation circuit 140) is operating in a burst mode, for example, asthe target intensity L_(TRGT) of a light source (e.g., the LED lightsource 202) is increased (e.g., from the low-end intensity L_(LE)). Asnoted herein, a control circuit (e.g., the control circuit 150 of theLED driver 100 shown in FIG. 1 and/or the control circuit 150controlling the forward converter 240 and the current sense circuit 260shown in FIG. 5) may adjust the average magnitude I_(AVE) of the loadcurrent I_(LOAD) by adjusting the active state period T_(ACTIVE) (i.e.,the number of inverter cycles per active state period T_(ACTIVE)). Whenadjusting only the active state period T_(ACTIVE) (e.g., and in turn theburst duty cycle DC_(BURST)) near the low-end intensity L_(LE), theadjustment of the average magnitude I_(AVE) of the load current I_(LOAD)may cause changes in the intensity of the lighting load that are visiblyperceptible to the user (e.g., as shown in FIG. 12A).

The control circuit may also adjust (i.e., increase or decrease) themagnitude of the load current I_(LOAD) between and/or during adjustmentsof the active state period T_(ACTIVE) while in the burst mode, forexample, to adjust the average magnitude I_(AVE) of the load currentI_(LOAD) with finer granularity as compared to adjusting only the activestate period T_(ACTIVE) (e.g., to provide finer tuning of the intensityof the lighting load). The control circuit may adjust (i.e., increase ordecrease) the magnitude of the load current I_(LOAD) by adjusting thetarget load current I_(TRGT) and by controlling the inverter circuit toregulate the load current I_(LOAD) to the target load current I_(TRGT)during the active state periods T_(ACTIVE), for example, as describedherein. The control circuit may adjust the load current I_(LOAD)linearly, variably as a function of the average magnitude I_(AVE) of theload current I_(LOAD), and/or by predetermined amounts. As such, thecontrol circuit may ease the transitions between adjustments of theactive state period T_(ACTIVE) by adjusting the load current I_(LOAD).

The control circuit may adjust the target load current I_(TRGT) by acurrent offset I_(OS), for example, between adjustments of the activestate period T_(ACTIVE) and/or when adjusting the active state periodT_(ACTIVE). The current offset I_(OS) may range (i.e., vary) between aminimum current offset I_(OS-MIN) and a maximum current offsetI_(OS-MAX), for example, based on the burst duty cycle DC_(BURST). Thevalue of the current offset I_(OS) may be determined based on theminimum rated current I_(MIN), the target current I_(TRGT), the targetintensity L_(TRGT), the burst duty cycle DC_(BURST), and/or the activestate period T_(ACTIVE). The current offset I_(OS) may be variablebetween burst mode periods T_(BURST) having the same active state periodT_(ACTIVE) and/or between burst mode periods T_(BURST) having thedifferent active state periods T_(ACTIVE).

Referring to FIG. 12B, the control circuit may increase the intensity ofthe light source at a constant rate. The control circuit may hold theactive state period T_(ACTIVE) of two or more burst mode periodsT_(BURST) constant, and may adjust the load current I_(LOAD) of theactive state periods T_(ACTIVE) of the two or more burst mode periodsT_(BURST), for example, by a consistent or varying current offsetI_(OS). For example, the control circuit may hold the active stateperiod T_(ACTIVE1) (i.e., the number of inverter cycles of the activestate period T_(ACTIVE1)) of the burst mode periods T_(BURST) 1212,1214, 1216 constant as shown in FIG. 12B. The control circuit may setthe load current I_(LOAD) during the active state period T_(ACTIVE1) ofthe burst mode period T_(BURST) 1212 to the minimum rated currentI_(MIN). The control circuit may increase the load current I_(LOAD) ofthe active state period T_(ACTIVE1) of the burst mode period T_(BURST)1214 by a current offset I_(OS-1) while holding the active state periodT_(ACTIVE1) constant. The control circuit may then increase the loadcurrent I_(LOAD) of the active state period T_(ACTIVE1) of the burstmode period T_(BURST) 1216 by a current offset I_(OS-2) while holdingthe active state period T_(ACTIVE1) constant. The current offsetI_(OS-2) may be greater than the current offset I_(OS-1). The currentoffset I_(OS-2) may be equal to, greater than, or less than twice thecurrent offset I_(OS-1). For example, the current offset I_(OS-2) may beequal to twice the current offset I_(OS-1) if the intensity of the lightsource is being increased at a constant rate. The control circuit mayincrease the average magnitude I_(AVE) of the load current I_(LOAD)during the burst mode periods T_(BURST) 1212, 1214, 1216 while holdingthe active state period T_(ACTIVE1) constant.

The control circuit may adjust the active state period T_(ACTIVE) andthe load current I_(LOAD) of a subsequent burst mode period T_(BURST).For example, the control circuit may increase the active state periodT_(ACTIVE2) of the burst mode period T_(BURST) 1218 and decrease theload current I_(LOAD) of the active state period T_(ACTIVE2) of theburst mode period T_(BURST) 1218. The control circuit may increase theactive state period T_(ACTIVE2) of the burst mode period T_(BURST) 1218by one inverter cycle, and may set the load current I_(LOAD) of theactive state period T_(ACTIVE2) of the burst mode period T_(BURST) 1218to the minimum rated current I_(MIN). Although the load current I_(LOAD)of the active state period T_(ACTIVE2) of the burst mode periodT_(BURST) 1218 is decreased, the average magnitude I_(AVE) of the loadcurrent I_(LOAD) is increased due to the increase in the active stateperiod T_(ACTIVE2). As such, the control circuit may control (e.g.,increase or decrease) the average magnitude I_(AVE) of the load currentI_(LOAD) with finer granularity by adjusting both the load currentI_(LOAD) and the active state period T_(ACTIVE) during burst mode.

When increasing the active state period T_(ACTIVE), the averagemagnitude I_(AVE) of the load current I_(LOAD) may be increased due tothe application of the load current I_(LOAD) for a greater duration ofthe burst mode period T_(BURST). For example, this may be illustrated by1250 in FIG. 12B. When increasing the load current I_(LOAD) during burstmode periods T_(BURST) having the same active state period T_(ACTIVE),the load current I_(LOAD) may be increased in excess of the minimum loadcurrent I_(MIN) by the current offset I_(OS) (e.g., which may varybetween the minimum current offset I_(OS-MIN) and the maximum currentoffset I_(OS-MAX)). As such, the average magnitude I_(AVE) of the loadcurrent I_(LOAD) may be increased due to the application of the currentoffset I_(OS) in excess of the minimum load current I_(MIN) for theactive state period T_(ACTIVE), for example, as illustrated by 1230 and1240 in FIG. 12B.

When determining the value of the current offset I_(OS) for a particularburst mode period T_(BURST), the control circuit may ensure that thechange (e.g., increase or decrease) in the average magnitude I_(AVE) ofthe load current I_(LOAD) due to the application of the current offsetI_(OS) does not exceed the change (e.g., increase or decrease) in theaverage magnitude I_(AVE) of the load current I_(LOAD) due to theapplication of the load current I_(LOAD) for a greater duration of timeduring the burst mode period T_(BURST) (i.e., an increase in the activestate period T_(ACTIVE)). For example, referring to FIG. 12B, thecontrol circuit may determine the current offset I_(OS-1) such that thechange in the average magnitude I_(AVE) of the load current I_(LOAD)1230 due to the current offset I_(OS-1) is less than the change in theaverage magnitude I_(AVE) of the load current I_(LOAD) 1240 due to thecurrent offset I_(OS-2). Similarly, the control circuit may determinethe current offset I_(OS-2) such that the change in the averagemagnitude I_(AVE) of the load current I_(LOAD) 1240 due to the currentoffset I_(OS-2) is greater than the change in the average magnitudeI_(AVE) of the load current I_(LOAD) 1230 due to the current offsetI_(OS-1) and is less than the increase in the average magnitude I_(AVE)of the load current I_(LOAD) 1250 due to the application of the loadcurrent I_(LOAD) for a greater duration of time during the burst modeperiod T_(BURST) (i.e., the difference between the active state periodT_(ACTIVE1) and the active state period T_(ACTIVE2)). This may allow thecontrol circuit may ease the transitions between adjustments of theactive state period T_(ACTIVE) (e.g., from the active state periodT_(ACTIVE1) to the active state period T_(ACTIVE2)) by adjusting theload current I_(LOAD) by a current offset I_(OS).

FIG. 12C shows example waveforms 1260, 1280 illustrating an example ofhow a load control circuit (e.g., the control circuit 150 of the LEDdriver 100 shown in FIG. 1 and/or the control circuit 150 controllingthe forward converter 240 and the current sense circuit 260 shown inFIG. 5) may determine the current offset I_(OS) when holding the activestate period T_(ACTIVE) constant during burst mode, for example, as thetarget intensity L_(TRGT) of a light source (e.g., the LED light source202) is increased from the low-end intensity L_(LE). As noted herein,the burst duty cycle DC_(BURST) (i.e., DC_(BURST-INTEGER)) and in turnthe active state period T_(ACTIVE) may be characterized by one or moreinverter cycles.

Referring to example waveform 1260, the active state period T_(ACTIVE1)of the burst mode period T_(BURST) 1262 may be characterized by twoinverter cycles 1266. In a subsequent burst mode period T_(BURST) 1264,the control circuit may determine to adjust the average magnitudeI_(AVE) of the load current I_(LOAD), for example, in accordance withthe target intensity I_(TRGT). For example, the control circuit maydetermine to increase the active state period T_(ACTIVE3) of the burstmode period T_(BURST) 1264 by less than one inverter cycle to achievethe increase in the average magnitude I_(AVE) of the load currentI_(LOAD). As such, the active state period T_(ACTIVE3) of the burst modeperiod T_(BURST) 1264 may be characterized by two inverter cycles 1266and a fractional portion 1268 of a third inverter cycle 1266, where forexample, the increase in the average magnitude I_(AVE) of the loadcurrent I_(LOAD) during the burst mode period T_(BURST) 1264 may be dueto the fractional portion 1268 of the third inverter cycle 1266.

As described herein, the ideal burst duty cycle DC_(BURST-IDEAL) of aburst mode period T_(BURST) may be characterized by an integer portionDC_(BURST-INTEGER) and/or a fractional portion DC_(BURST-FRACTIONAL).For example, the ideal burst duty cycle DC_(BURST-IDEAL) may follow theideal curve shown in FIG. 3 (e.g., the ideal burst duty cycleDC_(BURST-IDEAL) may result in the active state period T_(ACTIVE3)). Theinteger portion DC_(BURST-INTEGER) may be characterized by thepercentage of the ideal burst duty cycle DC_(BURST-IDEAL) that includescomplete inverter cycles. The fractional portion DC_(BURST-FRACTIONAL)may be characterized by the percentage of the ideal burst duty cycleDC_(BURST-IDEAL) that includes a fraction of an inverter cycle. Forexample, as shown in FIG. 12C, the ideal burst duty cycleDC_(BURST-IDEAL) of the burst mode period T_(BURST) 1262 may include aninteger portion DC_(BURST-INTEGER) 1270 (which may be characterized bytwo inverter cycles 1266), but not include a fractional portionDC_(BURST-FRACTIONAL). The ideal burst duty cycle DC_(BURST-IDEAL) ofthe burst mode period T_(BURST) 1264 may include an integer portionDC_(BURST-INTEGER) 1270 (which may be characterized by two invertercycles 1266) and a fractional portion DC_(BURST-FRACTIONAL) 1272 (whichmay be characterized by the fractional portion 1268 of the thirdinverter cycle 1266).

However, the control circuit may be configured to adjust the number ofinverter cycles only by an integer number (i.e., by complete invertercycles) and not by a fractional amount. Therefore, the control circuitmay be unable to increase the active state period T_(ACTIVE3) of theburst mode period T_(BURST) 1264 by the fractional portion 1268 and inturn increase the burst duty cycle DC_(BURST) of the burst mode periodT_(BURST) 1264 by the fractional portion DC_(BURST-FRACTIONAL) 1272 toincrease the average magnitude I_(AVE) of the load current I_(LOAD). Thecontrol circuit may adjust the magnitude of the load current I_(LOAD) bythe current offset I_(OS) to compensate for not being able to adjust thenumber of inverter cycles by a fractional amount, for example, asdescribed with reference to FIG. 15.

As noted above, during burst mode, the control circuit may increase theload current I_(LOAD) of an active state period T_(ACTIVE) by a currentoffset I_(OS) in order to increase the average magnitude I_(AVE) of theload current I_(LOAD) to achieve the target intensity L_(TRGT). Thecontrol circuit may determine the current offset I_(OS) based on thefractional portion DC_(BURST-FRACTIONAL) of the ideal burst duty cycleDC_(BURST-IDEAL) for the burst mode period T_(BURST), for example,assuming that the control circuit could in fact adjust the number ofinverter cycles by a fractional amount. For example, referring to thewaveform 1280, the control circuit may determine the current offsetI_(OS-1) based on the fractional portion DC_(BURST-FRACTIONAL) 1272 ofthe ideal burst duty cycle DC_(BURST-IDEAL) of the burst mode periodT_(BURST) 1264 from waveform 1260. That is, the control circuit maydetermine the current offset I_(OS-1) such that an increase 1274 in theload current I_(LOAD) over the active state period T_(ACTIVE) due to thecurrent offset I_(OS-1) may be equal to (i.e., result in the sameadjustment to the average magnitude I_(AVE) of the load currentI_(LOAD)) the fractional portion DC_(BURST-FRACTIONAL) 1272. Therefore,the control circuit may adjust the magnitude of the load currentI_(LOAD) by the current offset I_(OS) (e.g., current offset I_(OS-1)) tocompensate for not being able to adjust the number of inverter cycles bya fractional amount (e.g., the fractional portion DC_(BURST-FRACTIONAL)1272).

FIG. 13 is an example of a plot relationship between the target loadcurrent I_(TRGT) and the burst duty cycle DC_(BURST), and the targetintensity L_(TRGT) of a light source (e.g., the LED light source 202),for example, when a load regulation circuit (e.g., the load regulationcircuit 140) is operating in a burst mode and when the load currentI_(LOAD) of the light source is near the low-end intensity L_(LE). Graph1300 is an example plot of a relationship between the target loadcurrent I_(TRGT) and the target intensity L_(TRGT) of the light source.Graph 1310 is an example plot of a relationship between the burst dutycycle DC_(BURST) (i.e., the integer portion of the ideal burst dutycycle DC_(BURST-INTEGER)) and the target intensity L_(TRGT) of the lightsource. In the graph 1300 and 1310, the target intensity may range fromthe transition intensity L_(TRAN) to the low-end intensity L_(LE).

A control circuit (e.g., the control circuit 150 of the LED driver 100shown in FIG. 1 and/or the control circuit 150 controlling the forwardconverter 240 and the current sense circuit 260 shown in FIG. 5) maydetermine the magnitude of the target load current I_(TRGT) and/or theburst duty cycle DC_(BURST) during burst mode, for example, based on thetarget intensity L_(TRGT). The control circuit may determine the targetintensity L_(TRGT), for example, via a digital message received via thecommunication circuit 180, via a phase-control signal received from adimmer switch, and/or the like. The target intensity L_(TRGT) may beconstant or may be changing (e.g., fading) from one intensity level toanother. The control circuit may determine the burst duty cycleDC_(BURST) based on the target intensity L_(TRGT). After determining theburst duty cycle DC_(BURST), the control circuit may determine thetarget load current I_(TRGT) that may be used with the burst duty cycleDC_(BURST) such that the light source is driven to the target intensityL_(TRGT). The control circuit may determine the burst duty cycleDC_(BURST) and/or the target load current I_(TRGT) by calculating thevalues in real-time (e.g., as described with reference to FIG. 15)and/or retrieving the values from memory (e.g., via a lookup table orthe like).

The control circuit may apply a particular burst duty cycle DC_(BURST)(i.e., DC_(BURST-INTEGER)) for a range of target intensities L_(TRGT).The control circuit may determine the target load current I_(TRGT)across the range of target intensities L_(TRGT) for the particular burstduty cycle DC_(BURST), for example, according to a target load currentI_(TRGT) profile. The target load current I_(TRGT) profile may varylinearly across the range of target intensities L_(TRGT) for theparticular burst duty cycle DC_(BURST). The target load current I_(TRGT)profile that may be applied across the range of target intensitiesL_(TRGT) associated with a burst duty cycle DC_(BURST) may be differentfor different burst duty cycles DC_(BURST). For example, the target loadcurrent I_(TRGT) may be adjusted from the minimum rated current I_(MIN)to the minimum rated current I_(MIN) plus the current offset I_(OS), andthe current offset I_(OS) may range from the minimum current offsetI_(OS-MIN) to the maximum current offset I_(OS-MAX) based on the burstduty cycle DC_(BURST). For example, the larger the burst duty cycleDC_(BURST), then the smaller range of current offsets I_(OS) may be usedin the target load current I_(TRGT) profile, and vice versa. This may bedone because the minimum rated current I_(MIN) divided by the burst dutycycle DC_(BURST) (i.e., I_(MIN)/DC_(BURST)) may be larger at smallerburst duty cycle DC_(BURST) values. Further, this may be done becausethe user's sensitively to changes in intensity of the light source maybe increased at lower light levels and a more granular adjustment ofintensity of the lighting load may be desired at low-end.

Referring to FIG. 13, if the control circuit determines that the targetintensity L_(TRGT) falls within the range 1301, then the control circuitmay determine to set the burst duty cycle DC_(BURST) (i.e.,DC_(BURST-INTEGER)) to DC_(MAX), and the control circuit may determineto set the target load current I_(LOAD) according to the target loadcurrent I_(LOAD) profile 1321. The target load current I_(LOAD) profile1321 may range from the minimum rated current I_(MIN) plus the minimumcurrent offset I_(OS-MIN) to the minimum rated current I_(MIN) based onthe target intensity L_(TRGT). As noted above, the control circuit maydetermine the burst duty cycle DC_(BURST) and/or the target load currentI_(TRGT) by calculating the values in real-time and/or retrieving thevalues from memory.

If the control circuit determines that the target intensity L_(TRGT)falls within the range 1302, then the control circuit may determine toset the burst duty cycle DC_(BURST) to 1312 (e.g., which may be lessthan the DC_(MAX), and the control circuit may determine to set thetarget load current I_(LOAD) according to the target load currentI_(LOAD) profile 1322. Similarly, if the control circuit determines thatthe target intensity L_(TRGT) falls within one of the target intensityranges 1303-1307, then the control circuit may determine to set theburst duty cycle DC_(BURST) to one of 1313-1317 and determine to set thetarget load current I_(LOAD) according to one of the target load currentI_(LOAD) profiles 1323-1327, respectively. The maximum target currentfor consecutive target load current I_(LOAD) profiles may change by aconstant amount (e.g., as shown in FIG. 13) or may change by varyingamounts (e.g., increase as the target intensity range gets smaller).Further, more or less than seven burst duty cycles DC_(BURST) (i.e.,DC_(MAX) through DC_(MIN)) may be provided between the transitionintensity L_(TRAN) and the low-end intensity L_(LE).

When the LED driver is driving a high-power LED light source, the LEDlight source may conduct larger amounts of current through the LEDdriver, which may affect the operation of the LED driver when dimming tothe low-end intensity L_(LE) (e.g., approximately 1%). For example, thelarger current conducted by the high-power LED light source may causethe load current I_(LOAD) to overshoot the minimum rated current I_(MIN)at the beginning of each active state period T_(ACTIVE).

FIG. 14A shows an example waveform 1400 illustrating an overshoot 1402in the load current I_(LOAD), for example, when the LED driver iscontrolling a high-power LED light source. The overshoot 1402 may be theadditional current in excess of the target load current I_(TRGT). Forexample, the target load current I_(TRGT) is the minimum rated currentI_(MIN) in FIG. 14B. Stated another way, the overshoot 1402 may becharacterized by the rise in the magnitude of the load current I_(LOAD)above the target load current I_(TRGT) at the beginning of the activestate period T_(ACTIVE). Since the LED driver determines the burst dutycycle DC_(BURST) (e.g., and thus the length of the active state periodT_(ACTIVE)) using open loop control, the overshoot (e.g., the overshoot1402) may cause the LED driver to deliver more power to the LED lightsource than intended. In turn, the overshoot 1402 may cause the averageload current I_(AVG) to increase in excess of what was intended, andthus the intensity of the LED driver may be greater than intended (e.g.,greater than approximately 1%). In addition, the sharp rise in themagnitude of the load current I_(LOAD) at the beginning of the activestate period T_(ACTIVE) (i.e., the overshoot 1402) may cause audiblenoise (e.g., buzzing) in the magnetic components of the load regulationcircuit of the LED driver (e.g., in the transformer 220 and/or theinductor L226 of the load regulation circuit 240 shown in FIG. 5).

Accordingly, the control circuit of the LED driver may be configured tocontrol the rise time of the load current I_(LOAD) at the beginning ofeach active state period T_(ACTIVE). FIG. 14B shows an example waveform1450 illustrating control of the rise time of the load current I_(LOAD)at the beginning of each active state period T_(ACTIVE). The controlcircuit 150 may detect an overshoot of the load current I_(LOAD), forexample, based on the load current I_(LOAD) as determined by the loadregulation circuit, based on the sense voltage V_(SENSE), based on theload current feedback signal V_(I-LOAD), and/or the like. The controlcircuit 150 may determine to control the rise time of the load currentI_(LOAD) at the beginning of an active state period T_(ACTIVE). Forexample, the control circuit 150 may determine to control the rise timeof the load current I_(LOAD) to prevent the overshoot from continuing tooccur. In one or more embodiments, the control circuit 150 may determineto control the rise time of the load current I_(LOAD) at any time duringoperation and/or for any reason, for example, the control circuit 150may be preconfigured to control of the rise time of the load currentI_(LOAD) at the beginning of each active state period T_(ACTIVE). Thecontrol circuit 150 may determine the on times T_(ON) of the drivecontrol signals V_(DRIVE1), V_(DRIVE2) for controlling the invertercircuit of the forward converter 240 (e.g., using the load currentfeedback signal V_(I-LOAD)). The control circuit 150 may be configuredto increase (e.g., ramp up) the load current I_(LOAD) from an initialcurrent I_(INIT) to the target load current I_(TRGT) (e.g., the minimumrated current I_(MIN) or to the minimum rated current I_(MIN) plus thecurrent offset I_(OS)) over a ramp time period T_(RAMP), for example, asshown in FIG. 14B. For example, the ramp time period T_(TRAMP) may beapproximately 200 microseconds. In addition, the initial currentI_(INIT) may be approximately 40% of the rated minimum current I_(MIN),but could range from zero amps to slightly less than the rated minimumcurrent I_(MIN).

Since the magnitude of the load current I_(LOAD) is less than theminimum rated current I_(MIN) during the ramp time period T_(RAMP), thecontrol circuit 150 does not regulate the magnitude of the load currentI_(LOAD) in response to the load current feedback signal V_(I-LOAD)during the ramp time period TRAMP. After freezing the control loopduring the inactive state period T_(INACTIVE), the control circuit 150may maintain the control loop in the frozen state while the controlcircuit is adjusting the on times T_(ON) of the drive control signalsV_(DRIVE1), V_(DRIVE2) to ramp the load current I_(LOAD) up to thetarget load current I_(TRGT) during the ramp time period T_(RAMP). Afterthe ramp time period T_(RAMP), the control circuit waits for a wait timeperiod T_(WAIT) (e.g., approximately 200 microseconds) before beginningto regulate the magnitude of the load current I_(LOAD) during aregulation time period T_(REG). As such, the control loop may be frozenfor the duration of the inactive state period T_(INACTIVE), the ramptime period T_(RAMP), and the wait time period T_(WAIT), and the controlloop may be unfrozen (active) during the regulation time period T_(REG).Ramping up the load current I_(LOAD) during the active state T_(ACTIVE)of the burst mode may prevent the load current I_(LOAD) fromovershooting the minimum rated current I_(MIN) at the beginning of eachactive state period T_(ACTIVE). The wait time period T_(WAIT) may beused to allow for the load current I_(LOAD) to stabilize. In one or moreembodiments, the wait time period T_(WAIT) may be omitted.

FIG. 14B may not be to scale. For example, in one or more embodiments,the ramp time period T_(TRAMP) may be approximately one quarter of apulse width (e.g., approximately 200 microseconds), the wait time periodT_(WAIT) may be approximately one quarter of a pulse width (e.g.,approximately 200 microseconds), and the regulation time period T_(REG)may be approximately one half of a pulse width (e.g., approximately 200microseconds). The invention is not so limited, and the ramp time periodT_(RAMP), the wait time period T_(WAIT), and the regulation time periodT_(REG) may be in different proportions of the pulse width.

The target intensity L_(TRGT) may be associated with a target amount ofpower delivered to the electrical load. For example, although theexample illustrated in FIGS. 2 and 3 are with relation to targetintensity L_(TRGT), the control circuit (e.g., control circuit 150) maybe configured to adjust the target load current I_(TRGT) to control thetarget amount of power delivered to the electrical load and/or thecontrol circuit may be configured to adjust the burst duty cycleDC_(BURST) in response to the target amount of power when operating inthe burst mode. For example, the control circuit may be configured toadjust the burst duty cycle DC_(BURST) linearly with respect to thetarget amount of power when operating in the burst mode. Further, thecontrol circuit may be configured to operate in the burst mode when thetarget amount of power is less than a transition amount of power. Thetransition amount of power may be power delivered to the electrical loadwhen the electrical load is controlled to be the transition intensityL_(TRAN).

FIGS. 15A, 15B, 16, 17A, 17B and 18 are simplified flowcharts of exampleprocedures for operating a forward converter in a normal mode and aburst mode. The procedures of FIGS. 15A, 15B, 16, 17A, 17B, and 18 maybe executed by a control circuit of a load control device (e.g., thecontrol circuit 150 of the LED driver 100 shown in FIG. 1 and/or thecontrol circuit 150 controlling the forward converter 240 and thecurrent sense circuit 260 shown in FIG. 5).

FIG. 15A is a simplified flowchart of an example target intensityprocedure 1500 that may be executed when the target intensity L_(TRGT)is adjusted at 1510 (e.g., in response to digital messages received viathe communication circuit 180). The control circuit may determine if itis operating the forward converter in the burst mode at 1512 (e.g., thetarget intensity L_(TRGT) is between the high-end intensity L_(HE) andthe transition intensity L_(TRAN), i.e., L_(TRAN)≦L_(TRGT)≦L_(HE)). Ifthe control circuit determines it is not operating the forward converterin the burst mode (e.g., but rather in the normal mode) at 1512, thenthe control circuit may determine and set the target load currentI_(TRGT) as a function of the target intensity L_(TRGT) at 1514 (e.g.,as shown in FIG. 2). The control circuit may then set the burst dutycycle DC_(BURST) equal to a maximum duty cycle DC_(MAX) (e.g.,approximately 100%) at 1516 (e.g., as shown in FIG. 3), and the controlcircuit may exit the target intensity procedure 1500.

If the control circuit determines that it is operating the forwardconverter in the burst mode at 1512 (e.g., the target intensity L_(TRGT)is below the transition intensity L_(TRAN), i.e., L_(TRGT)<L_(TRAN)),then the control circuit may set the target load current I_(TRGT) to aminimum value (e.g., to the minimum rated current I_(MIN)) at 1518(e.g., as shown in FIG. 2). The control circuit may then determine andset the burst duty cycle DC_(BURST) as a function of the targetintensity L_(TRGT) at 1520 (e.g., using open loop control as shown inFIG. 3), and the control circuit may exit the target intensity procedure1500.

FIG. 15B is a simplified flowchart of an example target intensityprocedure 1550 that may be executed when the target intensity L_(TRGT)is adjusted at 1560 (e.g., in response to digital messages received viathe communication circuit 180). The control circuit may determine if itis operating the forward converter in the burst mode at 1562 (e.g., thetarget intensity L_(TRGT) is between the high-end intensity L_(HE) andthe transition intensity L_(TRAN), i.e., L_(TRAN)≦L_(TRGT)≦L_(HE)). Ifthe control circuit determines that it is not operating the forwardconverter in the burst mode (e.g., but rather in the normal mode), thenthe control circuit may determine and set the target load currentI_(TRGT) as a function of the target intensity L_(TRGT) at 1564 (e.g.,as shown in FIG. 2). The control circuit may then set the burst dutycycle DC_(BURST) equal to a maximum duty cycle DC_(MAX) (e.g.,approximately 100%) at 1566 (e.g., as shown in FIG. 3), and the controlcircuit may exit the target intensity procedure 1550.

If the control circuit determines that it is operating the forwardconverter in the burst mode at 1562 (e.g., the target intensity L_(TRGT)is below the transition intensity L_(TRAN), i.e., L_(TRGT)<L_(TRAN)),then the control circuit may determine the burst duty cycle DC_(BURST)and target load current I_(TRGT) for one or more burst mode periodsT_(BURST) (e.g., using open loop control) at 1568. For example, thecontrol circuit may determine the burst duty cycle DC_(BURST) and/or thetarget load current I_(TRGT) by calculating the values in real-time(e.g., as described with reference to FIG. 16) and/or retrieving thevalues from memory (e.g., via a lookup table or the like). The controlcircuit may determine the burst duty cycle DC_(BURST) and target loadcurrent I_(TRGT) for a plurality of burst mode periods T_(BURST) thatmay be used when adjusting the intensity of the light load to the targetintensity L_(TRGT), for example, as described with reference to FIG. 12Band/or FIG. 13. The control circuit may then set the burst duty cycleDC_(BURST) and target load current I_(TRGT) for each of the plurality ofburst mode periods T_(BURST) at 1570, for example, until the intensityof the lighting load equals the target intensity L_(TRGT), and thecontrol circuit may exit the target intensity procedure 1550.

FIG. 16 is a simplified flowchart of an example target load currentI_(TRGT) procedure 1600. The target load current I_(TRGT) procedure 1600may be executed periodically (e.g., every 66 microseconds). In one ormore embodiments, the target load current I_(TRGT) procedure 1600 may beexecuted during 1568 of the target intensity procedure 1550. The controlcircuit may determine the burst duty cycle DC_(BURST) (e.g., ideal burstduty cycle DC_(BURST-IDEAL), integer portion of the ideal burst dutycycle DC_(BURST-INTEGER), and/or fractional portion of the ideal burstduty cycle DC_(BURST-FRACTIONAL)), the current offset I_(OS), and targetload current I_(TRGT) for controlling the lighting load at the beginningof every burst mode period T_(BURST), for example, using the target loadcurrent I_(TRGT) procedure 1600. The target load current I_(TRGT)procedure 1600 may begin at 1602 where the control circuit may determinethe target intensity L_(TRGT). The control circuit may determine thetarget intensity L_(TRGT), for example, via a digital message receivedvia the communication circuit 180, via a phase-control signal receivedfrom a dimmer switch, and/or the like. The target intensity L_(TRGT) maybe constant or may be changing from one intensity level to another.

After determining the target intensity L_(TRGT), the control circuit maydetermine the ideal burst duty cycle DC_(BURST-IDEAL) at 1604. The idealburst duty cycle DC_(BURST-IDEAL) may be adjusted linearly as the targetintensity L_(TRGT) is adjusted between the low end intensity L_(LE) andthe transition intensity L_(TRAN). For example, the control circuit maydetermine the ideal burst duty cycle DC_(BURST-IDEAL) based on thetarget intensity L_(TRGT) using the graph of FIG. 3. The ideal burstduty cycle DC_(BURST-IDEAL) may include an integer portionDC_(BURST-INTEGER) and/or a fractional portion DC_(BURST-FRACTIONAL),for example, as described with reference to FIG. 12C.

The control circuit may determine the integer portion of the ideal burstduty cycle DC_(BURST-INTEGER) at 1606. For example, the control circuitmay determine the integer portion of the burst duty cycleDC_(BURST-INTEGER) by rounding the ideal duty cycle DC_(BURST-IDEAL)down to the next closest integer value using the following equation:

DC_(BURST-INTEGER)=Round-Down(DC_(BURST-IDEAL)).

The control circuit may determine the fractional portion of the idealburst duty cycle DC_(BURST-FRACTIONAL) at 1608. For example, the controlcircuit may determine the fractional portion of the ideal burst dutycycle DC_(BURST-FRACTIONAL) by subtracting the integer portion of theburst duty cycle DC_(BURST-INTEGER) from the ideal burst duty cycleDC_(BURST-IDEAL), for example, using the following equation:

DC_(BURST-FRACTIONAL)=DC_(BURST-IDEAL)−DC_(BURST-INTEGER).

As noted herein, the fractional portion of the ideal burst duty cycleDC_(BURST-FRACTIONAL) may be characterized by the percentage of theideal burst duty cycle DC_(BURST-IDEAL) that includes a fraction of aninverter cycle. And since the control circuit may be configured toadjust the number of inverter cycles only by an integer number and not afractional amount (i.e., by DC_(BURST-FRACTIONAL)), the control circuitmay determine the current offset I_(OS) for the burst mode periodT_(BURST) such that an increase in the load current I_(LOAD) due to thecurrent offset I_(OS) may be equal to (i.e., result in the sameadjustment to the average magnitude I_(AVE) of the load currentI_(LOAD)) the fractional portion DC_(BURST-FRACTIONAL).

The control circuit may determine the average current I_(DUTY) generatedduring a burst mode period T_(BURST) having a burst duty cycleDC_(BURST-CYCLE) that comprises one inverter cycle using the minimumrated current I_(MIN) at 1610. The minimum rated current I_(MIN) may bethe peak current when the target intensity L_(TRGT) is less than thetransition intensity L_(TRAN), for example, as shown in FIG. 2. Forexample, the control circuit may determine the average current I_(DUTY)that is generated during a burst mode period T_(BURST) having a burstduty cycle DC_(BURST-CYCLE) that comprises one inverter cycle, forexample according to the following equation:

I _(DUTY) −I _(MIN)/DC_(BURST-CYCLE).

The control circuit may determine the current offset I_(OS) according tothe amount of current I_(DUTY) generated during a burst duty cycleDC_(BURST-CYCLE) that comprises one inverter cycle and the fractionalportion of the ideal burst duty cycle DC_(BURST-FRACTIONAL) at 1612. Forexample, the control circuit may multiply the fractional portion of theideal burst duty cycle DC_(BURST-FRACTIONAL) by the average currentI_(DUTY) generated during a burst mode period T_(BURST) having a burstduty cycle DC_(BURST-CYCLE) that comprises one inverter cycle todetermine the current offset I_(OS), for example, according to thefollowing equation:

I _(OS) =I _(DUTY)·DC_(BURST-FRACTIONAL).

After determining the current offset I_(OS), the control circuit may addthe current offset I_(OS) to the minimum rated current I_(MIN) todetermine the target current I_(TRGT) for the target intensity L_(TRGT)at 1614, and the control circuit may exit the target load currentI_(TRGT) procedure 1600. Since the control circuit may be configured toadjust the number of inverter cycles by an integer number and not afractional amount, the control circuit may operate in burst mode byusing the integer portion of the burst duty cycle DC_(BURST-INTEGER),and by using the current offset I_(OS) in lieu of the fractional portionof the ideal burst duty cycle DC_(BURST-FRACTIONAL). As such, thecontrol circuit may control the load current I_(LOAD) to achieve thetarget intensity although it may not be able to operate at the idealburst duty cycle DC_(BURST-IDEAL).

In one or more embodiments, the control circuit may determine a scaledtarget intensity L_(SCALED) and use the scaled target intensityL_(SCALED) in lieu of the target intensity L_(TRGT) when determining theideal burst duty cycle DC_(BURST-IDEAL) during the target load currentI_(TRGT) procedure 1600. For example, the may determine and use thescaled target intensity L_(SCALED) when the low-end intensity L_(LE)and/or the minimum burst duty cycle DC_(MIN) are not zero (e.g., whenthe low-end intensity L_(LE) is approximately in the range of 0.1%-1%).The scaled target intensity L_(SCALED) may be based on the targetintensity L_(TRGT), the minimum burst duty cycle DC_(MIN), and themaximum burst duty cycle DC_(MAX). For example, the control circuit maydetermine the scaled target intensity L_(SCALED) using the followingequation:

$L_{SCALED} = {\frac{( {L_{TRGT} - L_{LE}} ) \cdot ( {{L_{TRAN} \cdot {DC}_{MAX}} - {L_{TRAN} \cdot {DC}_{MIN}}} )}{( {L_{TRAN} - L_{LE}} )} + {L_{TRAN} \cdot {{DC}_{MIN}.}}}$

After determining the scaled target intensity L_(SCALED), the controlcircuit may determine the ideal burst duty cycle DC_(BURST-IDEAL) basedon the scaled target intensity L_(SCALED), for example, at 1604, and thetarget load current I_(TRGT) procedure 1600 may proceed as describedherein. For example, the control circuit may determine the ideal burstduty cycle DC_(BURST-IDEAL) based on the scaled target intensityL_(SCALED) using the following equation:

DC_(BURST)=(L _(SCALED) /L _(TRAN))*T _(BURST)

FIG. 17A is a simplified flowchart of an example control loop procedure1700, which for example, may be executed periodically (e.g., every 66microseconds). The control loop procedure 1700 may begin at 1710. Forexample, the control circuit may execute the control loop procedure 1700to adjust the on time T_(ON) of the drive control signals V_(DRIVE1),V_(DRIVE2) (e.g., and thus the duty cycle DC_(INV) of the invertervoltage V_(INV)) in response to the magnitude of the load currentI_(LOAD) determined from the load current feedback signal V_(I-LOAD)received from the current sense circuit. The control circuit maydetermine if it is operating the forward converter in the normal mode at1712. If not, then the control circuit may determine if it is operatingthe forward converter in the active state of burst mode at 1714. If thecontrol circuit is operating the forward converter in the normal mode orin the active state of burst mode, then the control circuit may adjustthe on time T_(ON) of the drive control signals V_(DRIVE1), V_(DRIVE2)in response to the load current feedback signal V_(I-LOAD). For example,the control circuit may determine if the magnitude of the load currentI_(LOAD) is too high at 1716 (e.g., I_(LOAD)>I_(TRGT)). If the magnitudeof the load current I_(LOAD) is too high, the control circuit maydecrease the on time T_(ON) of the drive control signals V_(DRIVE1),V_(DRIVE2) at 1718. For example, at 1718, the control circuit maydecrease the on time TON by a predetermined amount or by an amountdependent upon the magnitude of the error between the target loadcurrent I_(TRGT) and the magnitude of the load current I_(LOAD). Afterdecreasing the on time T_(ON) of the drive control signals V_(DRIVE1),V_(DRIVE2), the control circuit may exit the control loop procedure1700.

If the control circuit determines that the magnitude of the load currentI_(LOAD) is not too high at 1716, then the control circuit may determinewhether the magnitude of the load current I_(LOAD) is too low at 1720(e.g., I_(LOAD)<I_(TRGT)). If the control circuit may determines thatthe magnitude of the load current I_(LOAD) is too low, the controlcircuit may increase the on time T_(ON) of the drive control signalsV_(DRIVE1), V_(DRIVE2) at 1722. For example, at 1722, the controlcircuit may increase the on time T_(ON) by a predetermined amount or byan amount dependent upon the magnitude of the error between the targetload current I_(TRGT) and the magnitude of the load current I_(LOAD).After increasing the on time T_(ON) of the drive control signalsV_(DRIVE1), V_(DRIVE2), the control circuit may exit the control loopprocedure 1700. If the control circuit determines that the magnitude ofthe load current I_(LOAD) is not too high at 1716 and is not too low at1720, the control circuit may exit the control loop procedure 1700.

If the control circuit is operating the forward converter in theinactive state of the burst mode, the control circuit may exit thecontrol loop procedure 1700 without adjusting the on time T_(ON) of thedrive control signals V_(DRIVE1), V_(DRIVE2). Accordingly, the controlcircuit may freeze the control loop when in the inactive state of theburst mode by not adjusting the on time T_(ON) of the drive controlsignals V_(DRIVE1), V_(DRIVE2) in response to the average magnitudeI_(AVE) of the load current I_(LOAD). If the magnitude of the loadcurrent I_(LOAD) is approximately zero amps during the inactive state,the control circuit may maintain the on time T_(ON) of the drive controlsignals V_(DRIVE1), V_(DRIVE2) (e.g., as stored in the memory 170) to beequal to the last value of the on time from the previous active state.The control circuit may control the magnitude of the load current to theminimum rated current I_(MIN) during the next active state.

FIG. 17B is a simplified flowchart of an example control loop procedure1750, which for example, may be executed periodically (e.g., every 66microseconds). For example, the control loop procedure 1750 may beexecuted by the control circuit to avoid overshoot in the load currentI_(LOAD) at the beginning of each active state period T_(ACTIVE) byramping up the load current I_(LOAD) (e.g., as shown in FIG. 14B). Thecontrol loop procedure 1750 may begin at 1760. The control circuit maydetermine if it is operating the forward converter in the normal mode at1762. If not, then the control circuit may determine if it is operatingthe forward converter in the active state of burst mode at 1764. If thecontrol circuit is operating the forward converter in the active stateof the burst mode at 1764, the control circuit may then determine at1765 if it should be presently regulating the load current (e.g., if itis operating in the regulation time period T_(REG) as shown in FIG.14B).

If the control circuit is operating the forward converter in the normalmode or in the regulation time period T_(REG) of the active state ofburst mode, then the control circuit may adjust the on time T_(ON) ofthe drive control signals V_(DRIVE1), V_(DRIVE2) in response to the loadcurrent feedback signal V_(I-LOAD). For example, the control circuit maydetermine if the magnitude of the load current I_(LOAD) is too high at1766 (e.g., I_(LOAD)>I_(TRGT)). If the magnitude of the load currentI_(LOAD) is too high, the control circuit may decrease the on timeT_(ON) of the drive control signals V_(DRIVE1), V_(DRIVE2) at 1768. Forexample, at 1768, the control circuit may decrease the on time T_(ON) bya predetermined amount or by an amount dependent upon the magnitude ofthe error between the target load current I_(TRGT) and the magnitude ofthe load current I_(LOAD). After decreasing the on time T_(ON) of thedrive control signals V_(DRIVE1), V_(DRIVE2), the control circuit mayexit the control loop procedure 1750.

If the control circuit determines that the magnitude of the load currentI_(LOAD) is not too high at 1766, then the control circuit may determinewhether the magnitude of the load current I_(LOAD) is too low at 1770(e.g., I_(LOAD)<I_(TRGT)). If the control circuit may determines thatthe magnitude of the load current I_(LOAD) is too low, the controlcircuit may increase the on time T_(ON) of the drive control signalsV_(DRIVE1), V_(DRIVE2) at 1772. For example, at 1772, the controlcircuit may increase the on time T_(ON) by a predetermined amount or byan amount dependent upon the magnitude of the error between the targetload current I_(TRGT) and the magnitude of the load current I_(LOAD).After increasing the on time T_(ON) of the drive control signalsV_(DRIVE1), V_(DRIVE2), the control circuit may exit the control loopprocedure 1750. If the control circuit determines that the magnitude ofthe load current I_(LOAD) is not too high at 1766 and is not too low at1770, the control circuit may exit the control loop procedure 1750.

If the control circuit is operating the forward converter in theinactive state of the burst mode at 1764, the control circuit may exitthe control loop procedure 1750 without adjusting the on time T_(ON) ofthe drive control signals V_(DRIVE1), V_(DRIVE2). Accordingly, thecontrol circuit may freeze the control loop when in the inactive stateof the burst mode by not adjusting the on time T_(ON) of the drivecontrol signals V_(DRIVE1), V_(DRIVE2) in response to the averagemagnitude I_(AVE) of the load current I_(LOAD). If the magnitude of theload current I_(LOAD) is approximately zero amps during the inactivestate, the control circuit may maintain the on time T_(ON) of the drivecontrol signals V_(DRIVE1), V_(DRIVE2) (e.g., as stored in the memory170) to be equal to the last value of the on time from the previousactive state.

If the control circuit is operating the forward converter in the activestate of the burst mode at 1764, but is not in the regulation timeperiod T_(REG) at 1765, the control circuit may adjust the on timeT_(ON) of the drive control signals V_(DRIVE1), V_(DRIVE2) using openloop control at 1774 to ramp up the load current at the beginning ofeach active state time period T_(ACTIVE) (e.g., during the ramp timeperiod T_(TRAMP) as shown in FIG. 14B). The control circuit may alsowait for the wait time period T_(WAIT) before once again beginning toadjust the on time T_(ON) of the drive control signals V_(DRIVE1),V_(DRIVE2) in response to the average magnitude I_(AVE) of the loadcurrent I_(LOAD) during the next regulation time period T_(REG).Accordingly, the control circuit may maintain the control loop frozenwhen not in the regulation time period T_(REG) in the active state ofthe burst mode. The control circuit may control the magnitude of theload current to the minimum rated current I_(MIN) during the nextregulation time period T_(REG).

FIG. 18 is a simplified flowchart of an example drive signal procedure1800, for example, which may be executed periodically. The drive signalprocedure 1800 may begin at 1810. The drive signal procedure 1800 may beexecuted periodically in accordance with the operating period T_(OP) ofthe inverter voltage V_(INV) of the forward converter 240. For example,the control circuit may execute the drive signal procedure 1800 togenerate the drive control signals V_(DRIVE1), V_(DRIVE2) using the ontime TON determined during the control loop procedure 1700 of FIG. 17Aor the control loop procedure 1750 of FIG. 17B. The control circuit maydetermine whether it is operating the forward converter in the normalmode at 1812. If not, then the control circuit may determine whether itis operating the forward converter in the active state of the burst modeat 1814. If the control circuit is operating the forward converter inthe normal mode or in the active state of the burst mode, the controlcircuit may determine whether the high-side FET Q210 or the low-side FETQ212 should be controlled at 1816. If the control circuit determinesthat it should control the high-side FET Q210 at 1816, the controlcircuit may drive the first drive control signal V_(DRIVE1) high toapproximately the supply voltage V_(CC) for the on time TON at 1818.

The control circuit may determine the magnitude of the load currentI_(LOAD) from the load current feedback signal V_(I-LOAD). The controlcircuit may determine if the target intensity L_(TRGT) is greater thanor equal to the threshold intensity L_(TH) at 1820. If so, the controlcircuit may set the signal-chopper time T_(CHOP) equal to the on timeTON at 1822. If the control circuit determines that the target intensityL_(TRGT) is less than the threshold intensity L_(TH) at 1820, thecontrol circuit may determine the offset time T_(OS) in response to thetarget intensity L_(TRGT) at 1824 (e.g., using one or more of therelationships shown in FIGS. 9 and 10). The control signal may set thesignal-chopper time T_(CHOP) equal to the sum of the on time T_(ON) andthe offset time T_(OS) at 1826.

Next, the control circuit may drive the signal-chopper control signalV_(CHOP) low towards circuit common for the signal-chopper time T_(CHOP)at 1828. The control circuit may sample the averaged load currentfeedback signal V_(I-LOAD) at 1830. The control circuit may calculatethe magnitude of the load current I_(LOAD) using the sampled value at1832. For example, the control circuit may calculate the magnitude ofthe load current I_(LOAD) at 1832 using the following equation:

${I_{LOAD} = \frac{n_{TURNS} \cdot V_{I - {LOAD}} \cdot T_{HC}}{R_{SENSE} \cdot ( {T_{CHOP} - T_{DELAY}} )}},$

where T_(DELAY) is the total delay time due to the turn-on time and theturn-off time of the FETs Q210, Q212 (e.g.,T_(DELAY)=T_(TURN-ON)−T_(TURN-OFF)), which may be equal to approximately200 microseconds. Finally, the control circuit may exit the drive signalprocedure 1800 after determining the magnitude of the load currentI_(LOAD).

If the control circuit determines that it should control the low-sideFET Q212 at 1816, the control circuit may drive the second drive controlsignal V_(DRIVE2) high to approximately the supply voltage V_(CC) forthe on time TON at 1834. The control circuit may exit the drive signalprocedure 1800 without the control circuit driving the signal-choppercontrol signal V_(CHOP) low or determining the magnitude of the loadcurrent I_(LOAD) from the load current feedback signal V_(I-LOAD). Ifthe control circuit determines that it is operating the forwardconverter in the burst mode at 1812 and in the inactive state at 1814,the control circuit may exit the drive signal procedure 1800 withoutgenerating the drive control signals V_(DRIVE1), V_(DRIVE2).

One or more of the embodiments described herein (e.g., as performed by aload control device) may be used to decrease the intensity of a lightingload and/or increase the intensity of the lighting load. For example,one or more embodiments described herein may be used to adjust theintensity of the lighting load from on to off, off to on, from a higherintensity to a lower intensity, and/or from a lower intensity to ahigher intensity. For example, one or more of the embodiments describedherein (e.g., as performed by a load control device) may be used to fadethe intensity of a light source from on to off (i.e., the low-endintensity L_(LE) may be equal to 0%) and/or to fade the intensity of thelight source from off to on.

Although described with reference to an LED driver, one or moreembodiments described herein may be used with other load controldevices. For example, one or more of the embodiments described hereinmay be performed by a variety of load control devices that areconfigured to control of a variety of electrical load types, such as,for example, a LED driver for driving an LED light source (e.g., an LEDlight engine); a screw-in luminaire including a dimmer circuit and anincandescent or halogen lamp; a screw-in luminaire including a ballastand a compact fluorescent lamp; a screw-in luminaire including an LEDdriver and an LED light source; a dimming circuit for controlling theintensity of an incandescent lamp, a halogen lamp, an electroniclow-voltage lighting load, a magnetic low-voltage lighting load, oranother type of lighting load; an electronic switch, controllablecircuit breaker, or other switching device for turning electrical loadsor appliances on and off; a plug-in load control device, controllableelectrical receptacle, or controllable power strip for controlling oneor more plug-in electrical loads (e.g., coffee pots, space heaters,other home appliances, and the like); a motor control unit forcontrolling a motor load (e.g., a ceiling fan or an exhaust fan); adrive unit for controlling a motorized window treatment or a projectionscreen; motorized interior or exterior shutters; a thermostat for aheating and/or cooling system; a temperature control device forcontrolling a heating, ventilation, and air conditioning (HVAC) system;an air conditioner; a compressor; an electric baseboard heatercontroller; a controllable damper; a humidity control unit; adehumidifier; a water heater; a pool pump; a refrigerator; a freezer; atelevision or computer monitor; a power supply; an audio system oramplifier; a generator; an electric charger, such as an electric vehiclecharger; and an alternative energy controller (e.g., a solar, wind, orthermal energy controller). A single control circuit may be coupled toand/or adapted to control multiple types of electrical loads in a loadcontrol system.

What is claimed is:
 1. An LED driver for controlling an intensity of anLED light source, the LED driver comprising: an LED drive circuitconfigured to conduct a load current through the LED light source and tocontrol the intensity of the LED light source; a current sense circuitconfigured to provide a load current feedback signal that indicates amagnitude of the load current; and a control circuit configured togenerate at least one drive signal for controlling the LED drive circuitto adjust an average magnitude of the load current, the control circuitconfigured to adjust an operational characteristic of the at least onedrive signal in response to the load current feedback signal in order tocontrol the intensity of the LED light source towards a targetintensity, wherein the control circuit is configured to operate in anormal mode to regulate the average magnitude of the load currenttowards a target load current that ranges from a maximum rated currentto a minimum rated current, the control circuit further configured tooperate in a burst mode to regulate the average magnitude of the loadcurrent below the minimum rated current; wherein, during the burst mode,the control circuit is configured to control the LED drive circuit in anactive state during a first time period in which the control circuitadjusts the operational characteristic of the at least one drive signalto regulate a peak magnitude of the load current towards approximatelythe minimum rated current in response to the load current feedbacksignal, the control circuit further configured to control the LED drivecircuit in an inactive state during a second time period in which thecontrol circuit stops generating the at least one drive signal andmaintains the operational characteristic of the at least one drivesignal constant, such that the average magnitude of the load current isless than the minimum rated current.
 2. The LED driver of claim 1,wherein the control circuit is configured to adjust a burst duty cycledefining when the LED drive circuit operates in the active state and theinactive state to adjust the average magnitude of the load current belowthe minimum rated current when operating in the burst mode.
 3. The LEDdriver of claim 2, wherein the control circuit is configured to adjustthe burst duty cycle in response to the target intensity when operatingin the burst mode.
 4. The LED driver of claim 3, wherein the controlcircuit is configured to adjust the burst duty cycle linearly withrespect to the target intensity when operating in the burst mode.
 5. TheLED driver of claim 2, wherein the control circuit is configured tooperate in the burst mode when the target amount of power is less than atransition amount of power.
 6. The LED driver of claim 2, wherein,during the burst mode, the control circuit is configured to regulate theaverage magnitude of the load current by adjusting the burst duty cycleand the target load current.
 7. The LED driver of claim 6, wherein,during the burst mode, the control circuit is configured to regulate thepeak magnitude of the load current to the minimum rated current duringthe first time period in a first burst mode period and regulate the peakmagnitude of the load current to the minimum rated current plus acurrent offset during the first time period in a second burst modeperiod.
 8. The LED driver of claim 7, wherein the current offset rangesfrom a minimum current offset to a maximum current offset based on theburst duty cycle and a target intensity.
 9. The LED driver of claim 8,wherein the control circuit is configured to increase the magnitude ofthe load current from an initial current to the minimum rated currentplus the current offset over a ramp time period at a beginning of thefirst time period.
 10. The LED driver of claim 1, wherein theoperational characteristic of the at least one drive signal comprises aduty cycle of the at least one drive signal.
 11. The LED driver of claim10, wherein the control circuit is configured to adjust the duty cycleof the at least one drive signal to regulate the average magnitude ofthe load current to the target load current in response to the loadcurrent feedback signal in the normal mode, the control circuit furtherconfigured to adjust the duty cycle of the at least one drive signal toregulate the peak magnitude of the load current to the minimum ratedcurrent in response to the load current feedback signal in the activestate of the burst mode.
 12. The LED driver of claim 10, wherein the LEDdrive circuit comprises a half-bridge inverter circuit, and the at leastone drive signal comprises two drive signals for driving the half-bridgeinverter circuit.
 13. The LED driver of claim 1, wherein the operationalcharacteristic of the at least one drive signal comprises an operatingfrequency of the at least one drive signal.
 14. The LED driver of claim1, wherein, after the second time period, the control circuit isconfigured to control the LED drive circuit in the active state during athird time period in which the control circuit generates the at leastone drive signal and adjusts the operational characteristic of the atleast one drive signal to regulate the peak magnitude of the loadcurrent towards approximately the minimum rated current in response tothe load current feedback signal.
 15. The LED driver of claim 1, whereinthe control circuit is configured to increase the magnitude of the loadcurrent from an initial current to the minimum rated current over a ramptime period at a beginning of the first time period.
 16. An LED driverfor controlling an intensity of an LED light source, the LED drivercomprising: an LED drive circuit configured to conduct a load currentthrough the LED light source and to control the intensity of the LEDlight source; a control circuit coupled to the LED drive circuit andconfigured to adjust an average magnitude of the load current, whereinthe control circuit is configured to operate in a normal mode and aburst mode, wherein the burst mode comprises an active state during afirst time period of a burst mode period and an inactive state during asecond time period of the burst mode period, and a duration of theactive state of the burst mode period is determined based on a burstduty cycle; and wherein, during the normal mode, the control circuit isconfigured to regulate the average magnitude of the load current byholding the burst duty cycle constant and adjusting a target loadcurrent, and wherein, during the burst mode, the control circuit isconfigured to regulate the average magnitude of the load current byadjusting the burst duty cycle and the target load current; and wherein,during the burst mode, the control circuit is configured to adjust thetarget load current by a current offset that ranges from a minimumcurrent offset to a maximum current offset based on the burst duty cycleand a target amount of power to be delivered to the LED light source.17. The LED driver of claim 16, wherein, during the burst mode, thecontrol circuit is configured to decrease the average magnitude of theload current by decreasing the target load current during consecutiveburst mode periods while holding the burst duty cycle constant, and bydecreasing the burst duty cycle and increasing the target load currentin a succeeding burst mode period.
 18. The LED driver of claim 16,wherein, during the burst mode, the control circuit is configured toincrease the average magnitude of the load current by increasing thetarget load current during consecutive burst mode periods while holdingthe burst duty cycle constant, and by increasing the burst duty cycleand decreasing the target load current in a succeeding burst modeperiod.
 19. The LED driver of claim 16, wherein, during the burst mode,the control circuit is configured to determine the current offset basedon a fractional portion of an ideal burst duty cycle.
 20. The LED driverof claim 16, wherein, in the normal mode, the control circuit isconfigured to regulate the average magnitude of the load current to thetarget load current that ranges from a maximum rated current to aminimum rated current, and wherein, in the burst mode, the controlcircuit is configured to control the average magnitude of the loadcurrent below the minimum rated current.
 21. The LED driver of claim 16,wherein the normal mode is characterized by the burst duty cycle beingequal to approximately 100%.
 22. The LED driver of claim 16, wherein thecontrol circuit is configured to stop regulating the load current duringthe second time period.
 23. The LED driver of claim 16, furthercomprising: a current sense circuit configured to provide a load currentfeedback signal that indicates the magnitude of the load current to thecontrol circuit; wherein the control circuit is configured to regulatethe average magnitude of the load current to the target load current inresponse to the load current feedback signal in the normal mode.
 24. TheLED driver of claim 16, wherein the control circuit is configured toincrease the magnitude of the load current from an initial current to aminimum rated current plus the current offset over a ramp time period ata beginning of the first time period.
 25. An LED driver for controllingan intensity of an LED light source, the LED driver comprising: an LEDdrive circuit configured to conduct a load current through the LED lightsource and to control the intensity of the LED light source; and acontrol circuit coupled to the LED drive circuit and configured toadjust an average magnitude of the load current, wherein the controlcircuit is configured to operate in a normal mode to regulate theaverage magnitude of the load current between a maximum rated current toa minimum rated current, the control circuit further configured tooperate in a burst mode to regulate the average magnitude of the loadcurrent below the minimum rated current; wherein, during the burst mode,the control circuit is configured to regulate a peak magnitude of theload current to a target load current during a first time period, and tostop regulating the load current during a second time period resultingin the average magnitude of the load current being below the minimumrated current, the control circuit configured to increase the magnitudeof the load current from an initial current to the target load currentover a ramp time period at a beginning of the first time period.
 26. TheLED driver of claim 25, further comprising: a current sense circuitconfigured to provide a load current feedback signal that indicates themagnitude of the load current; wherein the control circuit is configuredto generate at least one drive signal for controlling the LED drivecircuit to adjust the average magnitude of the load current, the controlcircuit configured to adjust an operational characteristic of the atleast one drive signal in response to the load current feedback signalin order to control the intensity of the LED light source towards atarget intensity.
 27. The LED driver of claim 26, wherein, during theburst mode, the control circuit is configured to adjust the operationalcharacteristic of the at least one drive signal to regulate the peakmagnitude of the load current towards approximately the minimum ratedcurrent in response to the load current feedback signal during at leasta portion of the first time period.
 28. The LED driver of claim 27,wherein the control circuit is further configured to stop generating theat least one drive signal and maintain the operational characteristic ofthe at least one drive signal constant during the second time period,such that the average magnitude of the load current is less than theminimum rated current.
 29. The LED driver of claim 28, wherein thecontrol circuit is configured to maintain the operational characteristicof the at least one drive signal constant while the control circuit isincreasing the magnitude of the load current from the initial current tothe target load current over the ramp time period.
 30. The LED driver ofclaim 29, wherein the control circuit is configured to adjust theoperational characteristic of the at least one drive signal to regulatethe peak magnitude of the load current towards approximately the minimumrated current in response to the load current feedback signal after theramp time period during the first time period.
 31. The LED driver ofclaim 30, wherein the control circuit is configured to wait for a waittime period after the ramp time period before beginning to adjust theoperational characteristic of the at least one drive signal to regulatethe peak magnitude of the load current towards approximately the minimumrated current in response to the load current feedback signal after theramp time period during the first time period.
 32. The LED driver ofclaim 25, wherein the target load current comprises one of the minimumrated current and the minimum rated current plus a current offset. 33.The LED driver of claim 32, wherein the current offset ranges from aminimum current offset to a maximum current offset based on a burst dutycycle and a target amount of power to be delivered to the LED lightsource.